Isn't the birth control pill supposed to stop ovulation

[misssarai]

I have been taking Yaz birth control for six months.  Other than some early bleeding (a few days before my period), I thought I seemed to be reacting well to it.  I have not been in a sexual relationship since I have started it; however I plan to enter into one soon with someone I am involved with.  Here is my question: a couple of weeks ago, I had a CT scan done (to check for kidney stones).  One of the findings on the CT analysis sheet was a "dominant follicule in the left ovary".  I looked this up online to see if it was something I should be concerned with, and what I read said that these are part of ovulation.  This caused concern, because I thought Yaz (as well as other BC pills) prevented ovulation?  Now I am wondering if I should be concerned that Yaz may not be working for me as well as it should?  I know it is a lower dose pill, so that's why I'm wondering if I should be concerned that it may not be working for me as well as it should, especially since I am planning on entering into a sexual relationship.  By the way, I take it around the same time every night and do not miss pills, and am not on any prescription drugs other than the occasional Xanax for a panic attack (not daily) or a Midrin for a migraine (also not daily).