13 days late... negative HPT and blood test

I'm 13 days late. I took a test 3 days before my period was expected, the day after it didn't show, 5 days after, and 2 days ago which was also when I had a blood test done by my doctor. Everything was negative. I have felt like my period was coming for the past 2 weeks, but there is no sign of it. I also have cramping, minor random headaches and very short bouts of nausea. I'm really confused because I have never been late before... I want to know what is going on. Has this ever happened to anyone? Where you pregnant or not? Does my ovulation date change now? Its supposed to be day 13 so I should be ovulating now... would I be or would it wait until I have my period? Ughh... so confused!!