What Can Be Cause For The Delay In The Periods?

Hello. I am 24 years old, slightly overweight 180 lbs at 5'5", but in good health. I have missed three periods and have taken about 7 home pregnancy tests, all of which have come back negative. In the past I have had problems with ovarian cysts, cervical cancer, and had gestational diabetes with my second pregnancy. I'm putting off visiting my doctor because I do not have health insurance. Occasionally I feel what seem like cramps, but they dissipate after a few days. I have also had some headaches, which is out of the norm for me as the only time I have experienced headaches was with my second pregnancy. Other than that I don't have any pregnancy symptoms, but I don't understand why I have no period!