What Causes Dark Brown Vaginal Discharge?

Hi, The last time I had my period was in March. It started on 2nd March, 2011 and lasted for about 5 days. Yes, I missed my period for 2 months but my period is never regular. I had missed my period for 6 months before but that was 2 years ago. I had unprotected sex with my boyfriend on 15th March, 27th March and the latest one was on 5th April. Around 2nd or 3rd May, I found some dark brown discharge instead of period. For 3 days, it was pretty heavy for a panty liner but too light for a pad and the rest of the days, panty liner is enough. There is also some dark red blood found when I wipe. My tummy hurts sometimes. I just want to know what problem it might be because I am a little worried. Hope you could answer my question. Thank you.