Dog yelps when peeing and pooping.?

our 9 yr old boxer has constant uti infections for about 8 months. she recovers well with zenequin. but now she is crying when she pees and poops. 2 vets found a m in the rectum but say it is not in the rectum but pushing up from underneath. they put her on rimadyl and she is her normal self. but we forgot a dose last night and she cried again this morning when she went to pee and poop. could the uti infection cause inflammation from below to push up into the rectum wall and cause the pain and m?