Hello,
If you have recently started with the pills then it takes around 3 months for the periods to come back to normal. It may be irregular for 3 months. If you have been taking the pills for quite some time, then it can be due to breakthrough bleeding due to hormonal effects of the pills.
Wait for some time and if the symptom persists then get it checked from a gynecologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
How long have you been on it? Sometimes it takes a few months on a new form of birth control before it all lines up. As it is, I've been on it for a little over a year and I still don't have my period until the 4th day on the sugar pills. But that's normal for me.
If you've only just gone on it I suggest seeing how it plays out next month. If you're still having problems spotting in between your period I suggest you talk to your dr about perhaps a different form of birth control.