Pregnancy matter?

Can somebody help me, just need some clarification. Actually I'm experiencing from Gastric ulcers last past weeks and I've been drinking 3 kinds of medecine 3 times a day till now. My last regular period starts on Nov 6, '07, I had an by Nov 18, and my period starts last Nov 30 but it starts brownish type and stops the very next day. I took a pregnancy test after two days and its negative result, I just worried why it stops, could it be because I'm taking medecine and I'm so i'll for the last 3 weeks? or there could be something else?