What Causes Light Vaginal Bleeding With No Regular Periods?

Last night my boyfriend and I had sex for the first time. I haven't had a regular period in about 7 months because I am very underweight - however, the last few months I have been getting some very light bleeding (no weight gain though). My last of the light bleeding was on about December 11 or 12. He wasn't wearing a condom (yeah I know, you don't have to tell me that it's bad, we won't do it again) and he pulled out WAY before he had to finish.