If you have been on the pill for 6 years and switched to a lower dosage 1 year ago what are the possible reasons for your period suddenly changing when it has been 100 predictable for those 6 years?
The lower dosage is likely a different synthetic compound which is having a different effect on you. That in itself is likely to be the reason for the change. If you had good results from your original pill with no problems why did you switch? Sounds as if you should go back to the old pill. Some women cannot tolerate the lower dose pills.