Your description doesn't sound like herpes. The negative blood test probably rules it out as well, although you don't say whether you were tested for HSV-2 only or also for HSV-1, which sometimes causes genital infection. Also, most routine STD testing doesn't include herpes, in which case your girlfriend's belief she is "STD free" may not mean she is free of herpes. Of course if she is infected, with or without symptoms, you could have acquired HSV from her since your blood test was done.
Still your description doesn't sound like herpes, which never recurs more often than every month or so; it cannot be responsible for a lesion that lasts only a couple of days then recurs after 3 days, with or without the trauma of vigorous masturbation. But if there is any doubt, you should check on what virus type(s) were included in your blood test and perhaps consider whether your girlfriend should be tested.
Good luck-- HHH, MD
Since you had negative herpes tests and your girlfriend is std free, it's not herpes. Maybe you masturbated too hard. Although genital herpes in usually mistaken for trauma, in your case it seems feasible. Or possibaly another skin disorder. Maybe you can consult a dermatologyst.
My Girlfriend and I have been together for 3 years and have been monogamous, except for me on the genital rubbing incident.
I havent had any scabs on my penis before this one, at least that i can remember.