Am I pregnant and experiencing implantation bleeding?

I've been on birth control for over a year now and my period is very regular. I usually never miss a pill until about 2 or 3 weeks ago, so the next day I took the missed pill and the pill of that day like you're supposed to. The last time I had my period was around the 17th of January. It only lasted 4 days. It usually lasts about 5 or 6 days. I had sex around the 25th or 26th. Not 100% sure of the date. I'm not due for my period for another day or 2, but the past week I have had some light pink/brown bleeding and light cramping. I thought my period was coming. But it has been very light. Not enough to use a tampon or anything. I see it most of the time whenever I wipe. Friday evening I got very nauseous and threw up. I hardly ever throw up! I have been very queasy and tired since then and today I have been extremely moody. I never had any sort of bleeding with my first pregnancy, so I'm not sure what's going on. I took a pregnancy test yesterday morning and it was negative.