Cramps but I had a brown period negtive pregnancy tests nd I take birth control what can be the problem?

Hormonal imbalance. Menstrual irregularities are not uncommon and anovulatory cycle is a common cause. If you miss more than two periods it would be time to see your doctor. If you do not wish to be pregnant, use contraception all the time. You may consider implanted contraceptive or an IUD. Consult this site for more information on this topic. Practice safe sex, if you have sex. Get HPV vaccine.