Progestin only pills=near constant period..

Emi
I've been taking the minipill for about 4 years now. At first it was great--i bled very little or not at all. Now, it's gotten to the point where I have a near constant, very light period (like 1 regular tampon-worth a day). I've had my period for 3 weeks this month. Nothing to make me feel anemic or anything. Has anyone else experienced this? What should I do?