I am very worried. This is my 3rd month on the pill (ortho tri-cyclen lo). I have never had spotting before, and I always take my pill at the same time every morning. However for the past week and a half I didn't take the pill at exactly the same time every morning, but I would at least in the same 3-hour time slot. Last Tuesday (12/20) I missed my pill, so I took it that same night when I remembered. The same day, my doctor prescribed me tetracyclin, an antibiotic... at the time I didn't know that antibiotics can interfere with the pill. On Thursday my boyfriend and I had sex, and within the hour afterwords I began spotting. At first it was pinkish, and it soon turned dark brown. It is very light. I am also having some cramps that are sharp and come and go very quickly. That night I stopped taking the tetracyclin because I realized that it interferes with the pill (I took a total of 5 of the tetracyclin pills). I have been spotting ever since we had sex. Today I took my last "active" pill (all of this has occurred within the 3rd week of my active pills), and my period is supposed to start in about 3 or 4 days. I took a home pregnancy test today that was negative, but I think I may have taken it too early.

After all that long scenario...
I am wondering if the spotting is most likely from the missed/late pill, taking the pills at different times of day, and/or the tetracyclin, OR if it is more likely that it is implantation bleeding from the sex we had the week before?