Depending on the situation, it is not normal to have your periods stop at 17, but if you are overweight it can happen for that reason. Periods can also stop because of pregnancy.
You say you have had sex twice -- do the missing periods correspond with having had sex? If so, have you taken a pregnancy test? And have you seen a doctor about the periods being AWOL?
I'd start with a home pregnancy test, and then move to a doctor appointment. Talk about the missing periods and talk about being overweight, and see what help you get.
Well, that's a long time to just be considered just irregular cycles. During the teen years, many girls do have irregular cycles. That's a long time though. I agree that you need to rule out pregnancy. You can get a pregnancy test at any drug store. Do that first and then you need to probably see a doctor. Planned parenthood is a place that usually will see teens without parents or if you are comfortable, tell your mom and make a regular doctors appointment. You don't have to have your mom in the room anymore at 17 and can talk to the doctor alone. Secondary amenorrhea is what they call it when a person has had a period and then stops. Polycystic Ovaries Syndrome is a reason that some girls gain weight and don't have periods. That's a medical condition that a doctor would diagnose. This can be a cause of secondary amenorrhea. So can thyroid issues. So, evaluation by a doctor would be best. Let us know!