Help please??

I had sex 3 weeks ago almost a month. About 4 days after sex I got my period. But now today about 5 days before my period I am spotting. Looks like it has clots.

-Would this be pregnancy implanation or just spotting before my period??

Never happened before.....

I took a pregnancy test 2 weeks after sex. It was negative.

I am on birth control. I am overweight if that does anything.