When my daughter was 3 she had this very thing and it ended up being a peri-urethral cyst. Basically, a cyst pocket had formed between the perineum and her urethra. She was complaining about her crotch hurting a little at the time, but since she was potty trained I just thought it was because of a lack of wiping or a bad wipe job. When I finally looked there was a brown sack bulging that appeared to come from her v*****. We had to go to a specialist but by that time it had broke itself and I had noticed more brown discharge at that time in her panties. My girls lack estrogen as newborns like most babies but it is more profound and causes there perineums to close (which I always check now). Hindset tells me that is what happened and the doctor nor I never noticed and it can close up and over the urethra. Other times if a little piece of tissue gets in the urethra it can also cause a cyst to form. Check her out down there. Little's are hard cause everything is so small but make sure there is no bulge and that her perineum is completely open. If it was a cyst you might be able to see the remains of it. If you do see a cyst or bulge get her into the tub with some goldenseal and garlic like a sitz bath.