The problem is not really 'being happy'. If your rabbit is really skittish, changing cage might have stressed him out which can cause a slowing of the guts and provoke a stasis (if he is currently moulting, the combination might have caused it). How long has it been going on? Is your rabbit showing signs of pain? (grinding his teeth, staying hunched or pressing his belly against the floor) Is he still pooping? (if you are unsure, empty the litter box so that you can monitor correctly what's going on). If he is, do the droppings look normal? Are they small?
If your rabbit is not pooping or making small poops (or worse, if there is mucus in it), you have to go to the vet now. And I mean NOW - phone your vet immediately and explain that it's an absolute emergency. You'll need an x-ray to check there isn't any blockage, and metacam for the pain and Emeprid (Reglan) to stimulate the guts if no blockage appears. Stasis is relatively easy to cure when caused by stress, if you take it early (the first 24hours are critical). It will cause death after a few days if left untreated.
