I agree it's probably worth a vet trip if her eating habits aren't normal for her.
It could be the junior pellets, you could be right, I'm not a massive fan of the adult Burgess pellets either, although some buns do fine on them, if you're getting sticky bum I would change from Burgess Junior to the adult Science Selective.
How many pellets do you feed? Excess pellets can also be a problem. Or have you introduced any new veg? It might not be agreeing with her xx