Month Long Periods

I`ve been on deprovera now for about four years and had no problems for the first while. I would just get a bit of spotting a few days before next needle was due just as others have said. But now just in the past year I have started having my period about a month before my next needle is due and have it for that full month each time. I am just wondering if anyone else has had this problem and/or knows if it`s a large problem or not. I`ve asked one doctor who said it`s not. It may not be a problem but spenting 1 out of every 3 monthes on my period is a problem for me!!! AHH! I`ve heard of many people having this in the first year but this is my fourth. Thats why it`s so odd.