What Causes Absence Of Periods?

I need to know what to do. September 18 I lost my virginity. It’s the only time that I have had sex. Two days after that I had a normal period. The next month a few days before I was suppose to get my real period I started spotting. It lasted only a few days then went away. The next month I completely missed my period. I then went to Planned Parenthood to get checked out. I don’t have anything and I’m not pregnant. The next month I got my period. I thought everything was fine but now I’m starting to get worried. I’ve been on my period for 24 days now and it is not showing any signs of letting up and time soon. Should I go to the ER or just go see a regular gynecologist?

What advice do you have for me?’

Thanks,

Karlie