Is anyone here a medical professional?

Not a big deal but I really like to know how my body works so if any one is a nurse or something do you know why this could be? I just started the pill and it has been working (I can tell by discharge and my hormone changes) however I started in the middle of the month instead of right after a period so theyre not lined up perfectly. Long story short this is when my period would normally be but my BC is not lined op with it so I am asking why would I have some breakthrough bleeding if I never produced and egg for my body to get rid of