Period on the implant

Basically I've had the implant for 3/4months now, I was on my Period when it was fitted. My normal period was every 4th week for 5 days. I know people can stop having periods and some have really heavy period. However mine is a funny brown discharge which comes on every 2 weeks and can last for up to a month. It's really gross and I can't have sex because I don't know when I'm going to come on. Should I go to the doctors or is this normal?