Hello,
Missed periods is the first sign of pregnancy. However to confirm the status of pregnancy, I suggest you to test for pregnancy by a highly sensitive pregnancy test 10 days post ovulation with a sensitive ( 20 - 25 mIU ) pregnancy test. Also then I would suggest you to get your serum HCG levels estimated since this is more specific than urine for pregnancy tests.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
There are quite a few possibilities. Most likely, you could be pmsing and right before you start a period OR it could also be form some of hormonal cause such as from a pregnancy or birth control.
If you are several weeks late on your period and know you're not pregnant I suggest you meet with a gyn to make sure there are no serious causes.