Hi Driver and welcome to GoY. In order to help you we need more info is the tree indoors or outdoors, if indoors just how warm is the area you are keeping it is and how are you watering?

Citrus trees do shed their leaves when they go through a period of stress. This can be caused by :-

1. Changes in temperature.

2. Not enough light.

3. Too much or too little water.

4. Poor ventilation.

5. Lack of humidity.

I suspect you kept your lemon tree in a conservatory over winter? If this is so, then lack of humidity is probably the most likely cause, especially as we are now starting to get a bit of sunshine which causes the relative humidity to drop in an enclosed space.

You could try standing the pot on an over sized saucer filled with wet gavel which will evaporate and provide humidity to the plant, and/or misting the leaves regularly.

Don't worry too much though, leaf drop is normal for citrus plants. It's usually just a way of them letting you know that their not happy, I'm sure it will survive and bounce back once the right conditions are provided for it again.