Excessive bleeding

I've had my Mirena for almost two years now. My periods have always been regular but ever since December they've been off. December I missed a period. January I was late and only had it for 4 days. And February I started, stopped 3 days later, started again 2 days later, that last a week. Stopped, and have now started again. My doctor said this could just be the start of my period ending but that doesn't sound normal to me. Any ideas?