Getting a Monthly Period on a Three Month Cycle Pill

I’m 5 months postpartum and this is the second time since starting a new pill that I’ve gotten a period when I’m not supposed to. Around 8 weeks PP I started back on the 3 month pill I was on before getting pregnant. But it was making me feel sick everyday so I went to the dr and they changed it to the less hormone option. Is it because there are less hormones that I’m all of a sudden getting a monthly period? Is it because my body isn’t back to normal yet? Should I try to go back to the other pill now that I’m 5 months PP? I wish this would stop it is beyond annoying on so many levels.