Period is 21 days late, what is going on?

My period is currently 21 days late. I am slightly overweight but my period has never been more than a couple of days late and I have never missed a period. I have a boyfriend and we have had unprotected sex (stupid I know) I have taken several pregnancy tests and they have all come back negative, even when using my first urine of the day. I haven't really got any pregnancy symptoms however I do have strange tummy pains & cramps, mild heartburn- on and off throughout the day and mood swings. When I first realised I was a week late I had felt very sick and did throw up once but there has been no sickness since. I am really confused at what is going on. Does anybody know what it could be and why it is happening?