Hi Shell, I was going to mention that it sounds suspiciously like a potential grass seed as well. Especially because there was a matt there to begin with. Grass seeds can often get caught in matts and then penetrate into the skin. Also, if you have lots of grasses or something that she walks or lays on, she may have developed an allergy. It's common on dogs bellies and genital regions when the new grasses start growing when the weather warms up.

Good luck and I hope everything is resolved quickly!