Why is my rats stomach swollen?

she has a bigger than normal stomache, bigger than it was this morning. she also has what looks like blood (brownish reddish color) around her tail and under her. it smells really bad. she ias somewhere around 1 1/2 years old. shes acting fine. shes walking around, crawling up the bars on her cage, i think shes eating and drinking. also, if a rat is sick, will healthy rats avoid her? because they arent. the other rats are both female. i thought i made a mistake and she was pregnant or somethine, but rats testicles are pretty big, and i dont see any on the other ones.