What Causes Irregular Bleeding With Pain While In Microlite?

Hi. I have a question about breakthrough bleeding. Ive been on the pill (microlite) for probably over a year now. I was put on it because of serious period pains - I used to pass out almost every time i got my period. Since starting the pill the pains are much easier to hande but the last few months they have been getting worse and the bleeding is more irregular :S this month i started bleeding 4 days before i finished taking the pill. It wasn't really like a normal bleed. It was very dark and it wasn't very noticeable. I now should have my normal period 3days but I've still just got this lite bleed. Its still vey dark,dry when i wipe and I'm experiencing cramps. Could you tell me why this is happening!? Is it normal or should I consider changing pills?