AF late after usually being normal, all bfn...any ideas?

I usually have a regular cycle (it has been getting shorter since getting off the pill about 6 months ago, leveling off at about 33 days) and we have been ttc for the last few months. I should have started on Monday and I started testing 2 days before that and every day since. I went to the doctor yesterday with a cold and went ahead and asked them to do a pregnancy test and it was negative as well. I'm not really sure what is going on, since I've always been pretty regular. And since I am not pregnant this month I am ready to get AF over and start my next cycle but it still hasn't hit. Does anyone have any idea what might be going on? Could I of had a chemical pregnancy or what could be causing this and what should I do? Thanks for any advice