Have you been watering regularly and has it suddenly been hotter or colder? If this was fresh potting compost you planted the seedlings in they shouldn't have used all the nutriments yet.

Do you think it might be a critter MG? I think I'd be checking very carefully for Red Spider Mites and White Flies further down the plants.

Moon, yes it was fresh and the weather hasn't changed significantly.
Cottage Karen, Actually the first time I grew chilies they were destroyed by spider mites, since then I have been treating my new chili plants with neem oil regularly and I keep checking the undersurface of the leaves, but I haven't found any till now.

After treating with the neem oil, the plants should be protected from direct sunlight, so I shifted them to my bedroom and somebody closed the windows and it was dark for a couple hours after which all the leaves drooped. They recovered back to normal the next day when they saw sun. Can that be a reason of a sudden change in environment for the plant?

There are 2 chili plants in the same pot. And both the plants are growing such curled up new leaves.

It could be Chillies...these heat loving plants are very sensitive to changes in temperature and light. HOpefully they will recover. I suggested Red Spider because that's exactly how my plants have curled with them in the past. So hopefully you will have got to them in time :) Good luck with them. I grew some one year and had so many fruits I ended up drying them off and using them for a couple of years. But to be honest, I'm not much of a chilli lover! :)

Well, in that case I would wait and watch since all the older leaves are still very full of life and perfectly normal.
Wow, I am so eager to get some fruits on this. Right now I have 5 chilli plants blossoming.
Btw, how did you fight off those spider mites, because with neem oil, I am not left with an option but to move them to a shady area for 3-4 days, so it gets a bit impractical and probably might have done the damage to the new leaves.
I am not a chili fanatic either, I just attempted to grow them and to my pleasure they have blossomed pretty well till now.
Thanks for the quick inputs :)

Hi Chillies...I am just resting so you got my attention:). OH has gone out to play golf and I am waiting for the sun to come out...oh, there it is now...so I can go and help my daughter in her garden. She won't be up yet though! Spider mites are just awful arent they? I had terrible problems with them in my conservatory (I no longer have a conservatory but I miss it). All I could do was hunt down an insecticide spray that said it controlled them (no idea if it had neem in it I'm afraid I can't remember what it was). But the trick is to get to them before they take over, and you've managed that by the sound of it. The first time I had them my poor Lemon Tree was dead before I realised. I was gutted. So I am very fastidious with the new Lemon Tree. Part of the answer it to keep the environment a bit moist. They cannot thrive where there is moisture. They love dry arid environments. This is tricky indoors but a bit easier in a greenhouse where you have to keep plants well watered. I used to put saucers of water on top of the radiators to try to keep some moisture in the air. I think that helped. Sometimes I put saucers of water on top of the pot compost so there was moisture in the air amongst the branches but not too much in the soil for plants that like to be kept dry. That seemed to help, but it takes a lot of maintaining.

Oh, I just noticed something terrible. The other chili plants are also starting to get curled leaves. Doesn't seem like it's insects/pests. The weather seems stable too. But there has to be a reason that all the plants are being affected at the same time.
Have fun with your garden karen, good day :)

Some varieties of chilies naturally develop bumpy leaves as they get ready to bloom and bear, especially some of the more tropical kinds, such as 'Habanero' and 'Bhut Jolokia'. If that is not the case, I would first carefully inspect the undersides of the leaves for aphids, and treat with soapy water, if needed.

I checked the images of those, this ain't any of that variety. The bloom and bear thing might apply to the bigger plants, but today I noticed that even the smaller ones have started to develop bumpy new leaves.
I frequently check the underside of the leaves. I don't see anything. But I think regardless I would treat them with soapy water. Maybe if some of the bugs are going unnoticed.
What composition of soapy water do you recommend?

Well, I noticed that the upper plant is showing signs of flower bud development, no bigger than a pinhead, yet. The two varieties that I mentioned aren't the only ones that show bumpy, twisted leaves around the inflorescence. 'Tabasco', "Chili de Arbol", 'Padron', and 'Marconi' also show that characteristic, as well as many others that I can't remember the names of, right now.
As for the soap, I use a liquid organic dish soap, about 15 ml to a liter of water. Using a bar soap is more difficult. I would use a mild one--not a harsh floor-scrubbing one--lather it up good, rinse it off with a stingy amount of water into a container, and dilute it until it is slippery feeling, but not particularly foamy, and put it into the sprayer. With either method, when in doubt, err on the side of caution: soap that isn't strong enough may only annoy the aphids, but at least it won't burn the leaves.

Seems like we have a good news.
I smelled the new leaves, and they have a slight pleasant and a slight chilli type hot smell. So, I guess your assumption is correct. Well you are a master gardener, it has to be correct.
I again checked thoroughly for aphids/ spider mites, no sign of them. Which means neem oil is doing it's job well.
thank you for mentioning the soap details, I will keep the info and use it when needed.
P.S: I am excited that my plant might start to flower soon :D