Implantation?

Patient: I had unprotected sex with my partner ejaculating about 3 weeks ago, a few days later I had my normal period, was on my period for one week, then off for one week. Now a little over a week I have been having a very faint light pink spot only a tiny bit at the same time for two days. I am on birth control, with my cycle every 3 months. I am slightly concerned that I may be pregnant, but the pregnancy test says negative. Is it too early?

Symptoms: spotting, weird feeling in lower abdomen (not cramping)

Doctor: