K.
FIrst, relax. It's probably temporary. Make sure you nurse him a lot and it will come back itself. Drink plenty etc. Chances are it will go back up in a few days. Try not to worry about it as worrying will make it worse. If you're running low on your freezer stash you can try pumping one breast while nursing him off the other - espeicially in the mornings, this will get you a lot of milk.
Second, you may be getting your period back... I've heard your supply drops a few days before it comes. (I wouldn't know - I got pregnant again 16 months postpartum without ever getting mine back!)