No AF!!!

OK as of today I am 7 days late. Taken loads of tests and all bfn so I'm not pregnant so what else could it be?? I've had no symptoms at all not even my usual PMS symptoms (no sore boobs). Occasionally I get dull cramps in the right ovary but that's been going on for 2 weeks now so can't be ovulation. What other reasons are there for missed periods?