If you mean just her nipples are enlarged, that will never go away. Females' nipples tend to get bigger after their first heat cycle. If she has swollen mammary tissue -- ie, looks like she had puppies -- she could very well be going through a false pregnancy. Some females will have them every heat cycle, and are usually good candidates for being spayed.
That is, of course, assuming that there is no chance that a male had access to her and that she could be pregnant. Only you would know if that is the case or not.
As for the persistent diarrhea, have you had her wormed recently? What are you feeding her? She may have a food allergy. Poor girl sounds like she's action-packed with issues. I would be asking a vet about all of these symptoms.