Rat pregnant from guinea pig?? Please help

Question
A few days ago I noticed that my female 6-month old rat has
suddenly gotten a gigantic stomach. It keeps getting bigger too.
It looks as if she's pregnant. I looked up pictures of pregnant
rats online, and she looks just like in those pictures. Thing is,
she lives with another female rat (which i KNOW for sure if a
female), and the only contact she has had with a male is my
guinea pig. We have had her for about 5 months now, so it is
impossible that she got pregnant from the pet store. In 5
months, she has only been with her sister and my guinea pig.
Would it be possible that my guinea pig made her
pregnant? I'm really worried about her because I don't know
what's happening. She's acting as happy as always and is in no
pain at all. Please tell me what could be wrong with her and if
indeed guinea pigs can get rats pregnant. Thank you! Nicole

Answer
Firstly this is NOT possible.  Guinea pigs and rats have different genes and baby guinea pigs would not fit in a rat!!!

Are you sure the other one is female?  If yes then it could be something serious such as bloat or obstruction of the bowel with a foreign body.  I suggest you take her to a vet to check it out