Why has my period been going on for a month?

I got my period about a month ago and I still have it. I use tampons if that means anything. Nothing has changed in terms of diet. Usually my period lasts a week. At first it was a normal period, but for the last three weeks it has been light-ish, so it doesn't bother me a lot. But my mom's worried as to why it hasn't stopped yet. She wants to bring my to a gynecologist. What will the doctor do? Thanks.