I have a betta in a planted Spec. He seems OK in the tank, but he keeps cramming himself in one corner, shimmying, and kicking the Aquasoil all over the place. He is active, healthy looking, eats just fine, water conditions are great... I don't know if he's stressed because the tank is too small, or if he's just a weird betta.
I've never had one dig before, but the one I have right now used to wedge himself under a piece of driftwood...it looked so uncomfortable, but he did it anyways and was/is perfectly healthy still. I've come to accept that bettas are just strange creatures...yours is probably not sick.
Bettas sometimes dig. If you've got a digger, you might want to be sure he can't get himself stuck under anything. If you've got rocks or driftwood, make sure it is all the way to the bottom of the tank and not on top of the substrate so he can't get underneath it.
Mine doesn't dig but she does like squeezing herself inside tight spaces in my tank - I often find her inside my x-mas moss, under a wedge in my driftwood, etc. It's almost like she sleeps there, from what I can tell