Two Weeks Late but Negitive Test Results?

I have been TTC for over a year. My period is now over two weeks late (which it has never been late, ever. I know to the day when it is going to start) and I have taken at lease seven home pregnancy tests that have all shown negative. After a week of my missing period, I called my doctor for a blood test. I got my results back that day which were also negative. I am tired all the time, I have cramps like I should be expecting my period, but it never comes. I am nauseated, have back pain, my breasts feel heavy, and I am constipated (which I have never been before). From all the research I have done, many say that women may not show a positive test until almost being four MONTHS pregnant due to lower hormone levels. This is just so unusal for my body... I have never been pregnant before so I don't know what exactly to expect to feel like, I just know what is normal for my body. Is there a chance I am pregnant? I am planning on scheduling an ultrasound if I miss my next period. I am not under anymore stress than I usually am, plus why would I be having all these weird symptoms? Can anyone give me some encouraging words?????