It is painful when I get a blowjob.

Last night my girlfriend was giving me a blowjob and it started becoming painful. Anyway, during it she stopped because I was in pain and I noticed a small purplish bump on the head. It looked like a blood blister. It was painful to the touch. Anyway it is still there and the head of my penis is sore, especially in that area. It isn’t really a bump today though, just a small purplish mark. It kind of looks like a small bruise. I don’t think it is an STD; she said she is clean and I was a virgin prior to her. We were first intimate four days ago. The first few times were unprotected, yet we didn’t actually go very far (I didn’t ejaculate). Before this last blowjob, I had received two from her. The first one felt great, although there was a little pain. The second one was kind of a mixture between pleasure and pain. The last one had quite a bit of pain. I don’t know if it is an irritation, if she nicked me with her teeth, or if she sucked too hard. I should point out that I bruise relatively easily and she told me I had a very sensitive penis. I would appreciate any help that anyone could give me.