Do you keep her w/ or around other males? If so, then she’s more than likely pregnant. Does go into heat about every 4 days or so. And if she is pregnant, as she gets bigger, her belly will be round and hard, and her nipples will be more noticeable. A rats’ gestation period(pregnancy) is about 21 days. And they will literally look like they swallowed a tennis ball by their last week! I would also HIGHLY suggest researching on google for information on the care of pregnant rats if you’ve never delt with this before. There’s more to it than you’d think.