What To Do With An Irregular Period?

Uh? Yeah, Hi. I'm 16 & I have an irregular period. I first got my period when I was 12. It was regular for the first 6 months the afterwards it went crazy. I now get it maybe once or twice a year. When I get my period it's very light maybe three days by barely. However, last October it was for five weeks straight. It wasn't always a problem. It became a big thing when I started to be sexually active. What can this be? Is it normal? Should I get this checked?