My Bunny has what looks like soap bubbles under her tail. What Causes that?

Well recently our heater in the house broke, so My mom is temporarily bunny Sitting. I ran out of the bale of hay that I had and had pelleted hay stashed for emergency. So she is eating that for the time being. It only started happening after she changed her diet. Also Because The big bag of bedding that we use for her is too big to fit in teh little truck we have, her bedding is news paper. Could it be the change in diet that caused this, or maybe that she is eating the news paper? All help is greatly appreciated.