Female bunnies sometimes do that even if they're not pregnant. They should bring her to a vet to be sure, and see about getting her spayed as well. But if they are COMPLETELY SURE she has encountered no other bunnies in two months (as in - if she has always been indoors the whole time), then she's not pregnant, as their gestation period is generally 35-40 days. How old is the bunny? Have they checked the skin to make sure there's no sign of fleas or anything?

Females bunnies, if not spayed, tend towards health problems, including ovarian cancer, so even if she is an "only bunny" and will never meet another one, it's best to "fix" her if she's old enough.