Breakthrough bleeding while on pill?

Eden
I've been on the same birth control pill for two years and I have noticed the last three months I've been spotting the first week after my period. It's varied on which day but it's always in the first week after my period. This month it was more than the last three months. 
I always worry it's pregnancy, but maybe it's just time to switch to a new pill. 
Does this happen to anyone else ?