Help?

So I've been really lost on this situation. I had unprotected sex during my fertile window (February 6,7, and 8th) and also February 4th. Well since Thursday ( February 15th) I've been spotting very very light pink

I talked to a few of my friends and they believe that I'm implantation bleeding. The light spotting stopped on the 18th and it's turned to a very light red but it's heavier but not even close to how heavy my normal period is.

I'm always tired, my back hurts, I don't have a sex drive and my lower abdomen hurts like mild cramps. I've taken a pregnancy test on the 16th and the 18th and they're both negative. What should I do? I read online once you're fertile it mocks your period?? But on implantation bleeding it states it that it stay a very light pink or a brown.