I agree with naturalremedy.. stress can cause your period to be delayed in starting. The more you can do to decrease your stress level, the better. While there are some things you cannot change, you can change the way in which you allow them to affect you.
Some suggestions for ways to relieve that stress is to get physical activity going, move that body and clear your mind. If you do not have a regular workout routine, you can always just put on some great music and dance your heart out right in your own living room. That is one of my favorites. If it is work related, make a point of leaving it at the office when you leave.
Other good options are to keep a clean diet, keep well hydrated and get plenty of sleep. Since you have had so many tests, you can clear your mind of possible pregnancy, and soon your period will start and live will return to normal. I wish you the best.
Feel free to post again with any other thoughts or concerns. We are here to help.
Depending on your age, this could also be a sign of changing hormones. Not knowing your age, I could be way off on this one, excuse me if I am off base.