Questions to ask yourself:

How often have I been watering it?
Does it have good drainage?
Is it in a large enough pot?
What have I been feeding it?
Did it turn color right after I fed it?
Is it planted in potting compost, or "soil conditioner"?
When I look really closely and carefully, are there any signs of bugs, webbing, or "scribble marks" on the leaves (especially the undersides) and young stems

Answer those questions, and you will be halfway to solving the problem.

I would suspect lack of watering...

That was my first suspicion, too, but with my experience with dahlias, I figured that it's worth checking everythng.