Well it is probably due to the medication especially if that is a known side effect. So I would not worry so much about it, since you have tested and know you are not pregnant. I would use birth control and condoms during intercourse in the future so you can out rule pregnancy and don't have to worry about becoming pregnant. As for your period, contact your doctor. He will probably put you on some progesterone like provera or something to help regulate your cycles. It depends on what he thinks. Maybe he will just have you wait and see what happens. But definitely use birth control and if you want to regulate your cycle then ask your doctor about progesterone (provera). I am a medical assistant and that is what they usually give people to regulate their cycles. Good luck.