Period is late, but I'm not pregnant???

My period is now 11 days late. I took 2 pregnancy tests yesterday. Both were negative (although I had sex on my period 2 months ago w/a condom and got my regular period last month). I've also been getting all my regular symptoms of a period just without the bleeding. The only other reason that I can think of for a late period is stress, but I'm not stressed. Is there anything else that could be delaying my period?