Mini pill, breastfeeding and period help?!

I exclusively breastfeed, she eats on demand which is every 1 1/2 - 2 hours from 10 - 20 minutes at time. I started the minipill on Saturday and started spotting. Well as of yesterday, Thursday, it’s a full blown period. I take it at 7am every day have never been late to take it. My OB told me I wouldn’t have a period with those two factors. And obviously he was wrong. Can anyone tell me how long I should expect this period to last? And has this happened to anyone else?