as pointed about, it can be caused by excessive scratching. If the skin has broken and you continue to scratch, it can cause an infected lesion, which is what it sounds like to me.
If the doctor has advised that you are negative for herpes and other STD's then I really wouldn't worry about it.
Just try smearing some anti-septic cream on the area and try not to touch it for a few days. Masturbation is fine, providing you try to avoid the inflamed area.
Hope this helps,
Adam
So the dr swabbed the sores and it came back neg for herpes??? If you didn't get it swabbed please do so!
If your scratching the area pretty hard then you could be causing your sores,
I'm not sure what it could be.