I have anxiety so sometimes I get anxious if my pill is working correctly. Im now wanting to gain more education on the topic so that I can feel more comfortable and I hope you can point me in the right direction.

I'm on Ortho Tri-Cyclen birth pills for 9 months. I get breakthrough bleeding/spotting every month in my last week of active pills. I've been told that this is a common side effect and that my lining just isn't being stabilized enough which is why I'm probably bleeding a bit but that I'm still fully protected.
My question Is, if that's happening to me then I would think that it's possible that the pill isn't working properly in preventing pregnancy in other ways such as stopping ovulation etc? I was told I'm wrong about this but I was wondering if I could have more in depth insight or even an article explaining it to me. I really appreciate It!