It could be any number of things, but if your DD is still gaining weight and is happy and sleeping well, it may be that there is nothing wrong with your supply but simply that your breasts don't "feel" as full?

I know with my DD and DS that I lost the "full" feeling at around 3.5 months, but that they still had plenty of wet nappies and were still sleeping well and were content.

Fennel tea and Fenugreek tablets can do great things for your supply, have you tried either of those?