Hello, Welcome to Just Answer and thanks for your question. If you are having pain, you need to have a medical exam. If you normal period started April 16, you would be fertile at ovulation, which is about 14 days before the next period is due. If your periods are regular and 28 days apart, ovulation would be April 30 and the next period would be expected May 14. The best time to take a pregnancy test is at least a week after a skipped period (May 21). Any period that lasts more than 10 days has to be evaluated since it can cause anemia to lose so much blood.
That doesn't alter things a lot - just a few days. There is next to no chance that you are pregnant.