Help! 16 and need some answers.

I'm 16 years old.  I had unprotected sex multiple times while ovulating without realising.  This is now two weeks ago, three weeks tomorrow.  I'm late for my period by 5 days.  I'm usually on time.  I've taken 2 pregnancy tests (one on Tuesday night and one when I woke up this morning which is Thursday) but both are negative.  I'm getting a few symptoms but I think that's just my head messing around with me because I think I might be pregnant? Can someone please explain why this is happening?