My sweet rat Chloe had developed a large mass that was pushing tautly against her vulva from the inside. I had only recently figured out that she was in pain. Fortunately, she is home from surgery now and medicated.
Update Dec. 29:
Chloe died 3 weeks ago. The tumor we had removed had regrown (as we expected it to) and had breeched some part of her uterus or vaginal such that she was bleeding heavily and she was in pain that the meds couldn't control.