What Causes Spotting Between Periods?

i am on noriday mini pill for 4 months now and never had any problems yet , but this month i have been bleeding in-between cycles . i only got my period 22 july for 4-5 days( which is normal for me ) but have been spot bleeding since ??? i was wondering if this is something to be concerned about as i missed 2 days of taking the pill ( the 23rd and 24th) when on my period ??? i am concerned it could be something with my cervix as i have had check ups for this ?
thank you