confused

Last month I had unprotected sex on the 8th of dec. then I missed my period, that was two weeks later and only had heavy spotting.(for two days and it was barely less than enough to wear a liner) I was real concerned about being pregnant so I took two pregnancy tests and they were both negative. Now I'm due for my next period and it is very very very light spotting. I am really confused about my body... any ideas?