Help with my 3 year old daughter's l pain?

At first she was diagnosed with a UTI and treated. After her last checkup to make sure it was gone, it was, but she is still complaining of pain. They have done a sonogram of her kidneys and bladder, which came back negative for any problems. Yet she is still complaining of pain. Sometimes she says her back hurts, but mostly she says "it hurts down there." She had a rash, which we treated with special steriod cream and is now gone. But when I just changed her, after having a bowel movement, I noticed her perineum looked a little swollen and her is as well and a bit red. She is also scheduled for a VCUG, but I'm concerned with the perineal pain she is having. Does anyone have any idea of what could be going on? We are at a loss. Also, her appetite is not great and she is not as active as she once was. This has been going on since the beginning of September. I am getting really worried!