Whats WRong With MY RAT!!!!!!!!!?

my rat bit my sister 2!! she never bit and she was soooooooo gentle!! this is my third week having her and her behavior is changing.... she used to be rowdy and run around and be energetic but now she is bitting and we are scared to take her out. She is getting very round and we askedd the pet store about her "change" and they said she maybe pregnant or just "heat"as in her period..... she never acted like this and my dad said if she isnt pregnant than we r giving her away...... i dont care if u think she isnt pregnant... it happens so just say what u really think is wrong with her