I was suppose to have my period on the 16th April. On the 18th April, I had some bleeding and then it stopped during that day. I'm now 18 days late and I have taken 3 pregnancy tests and a negative?

Hormonal imbalance. If you did the tests correctly and the result was negative at 18 days after missed period, you are unlikely to be pregnant. 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. http://www.nichd.nih.gov/health/topics/menstruation/conditioninfo/Pages/causes.aspx Practice safe sex. Get HPV vaccine.