Missed Pd, discharge, neg test.

So Dec 25 was the last time I had my period. I ovulated the first week of Jan, had sex the 6th and the 9th. Was due for my pd on Jan 21st. A couple of days before my pd, I notice some mild cramping like I was going to start. As the 21st creeped on by I still had cramps on and off, but then came some unusal discharge. This charge was thick white then clear, and had no smell. It comes and goes just like the cramps. I have started to get headaches and heartburns. I went to the store and bought a pregnancy test but was neg. There are times where I think I have to do the number 2, then when I get to the bathroom I'm sitting and waiting and nothing comes. Yesterday I took a test and it came back neg as well. So I'm going to the dr Thursday. Just wonder what it could be. It's been 3 weeks and no period.