sick betta, any ideas?

Hi this is my first post.
My betta has been very happy in the last three months or so since i got him, swimming and eating well.
For the last four or five days he has been spending all his time on the bottom of the tank hiding between the rocks and generally looking very sorry for himself.
I cant actually see any signs of whats wrong with him, no spots etc.
The only thing i can come up with is that the last time he ate he stuffed himself, too much.
Hes the eyes bigger then his belly kind of fish lol.
I was wondering if he might be constipated or something.
Im just looking for some ideas of how i can help him, i dont like seeing him laying around like he is.
Any help would be gladly recieved.
Thanks