I had sex with my boyfriend February 4th, and we used a condom and he pulled out with the condom on prior to ejaculation. The condom had not broken. We then had oral sex and, as he moved, a drop of ejaculate fell on the outside of my vagina; I believe some may have went in, but I am not positive. Therefore, I took Plan B- both pills at the same time- about 15 hours later. The only problem is that I am also on antibiotics- isoniazid, or INH, which is supposed to decrease the effectiveness of a birth control pill. I am not sure about its effects on Plan B, however. My period started on January 25th and ended around February 1st. I had dark brown spotting for about three days starting on February 16th. On February 26th, when I would wipe, there were tiny dots of pink blood. For 8 or 9 days after, I had dark brown bleeding- some of which was heavy and took up an entire tampon- with red and pink blood only sometimes. It was almost always dark brown. I am not sure whether or not this was my period, either, since it occurred around the time it was due. My breasts were sore earlier in the month of February, but are no longer tender at all. What are the chances that I could be pregnant? Please help.