Since she's a girl, it's possible that her "fatness" was due to eggs. Since she wasn't bred, she may have reabsorbed them. (Or she may have released them and eaten them.) When this happens to females, they generally become less "chubby" and their poop may be white for a few days.
I recently had this happen with my girl. She looked bloated, but her behavior didn't change. She released the eggs, and ate a bunch. (I removed the rest, once I saw what was going on.) For the next day or two, her poop was much lighter-colored than normal. She now looks like she's been on a diet (even though she's eating as much as ever), and her activity level is the same as usual.
If you notice anything strange going on though, definitely tell us. If the bloating and light-colored poop don't go away in a day or so, or if her behavior or appearance changes in any way, this may indicate a parasite issue or some other problem.