If it's not a false negative pregnancy test and you truly aren't pregnant, then the reason for the late period and pregnancy-like symptoms could just be prolonged PMS. Anecdotally, my PMS symptoms always worsen the later I get.
You could also be dealing with a hormonal imbalance or a thyroid condition, both of which are pretty treatable with a doctor's care.
For now, I'd stop taking pregnancy tests and give it a few days. If your period becomes two weeks late, call your doctor and let them know. I have a friend who couldn't get a positive on her pregnancy test until she was two weeks late!