Ovulation

I am extremely confused with how ovulation works. I have read and read and no understanding sticks. Anyone can explain?... How do you know? On CD 10 I had a SIS test and doctor and medical assistant were wowed with my anatomy. I have a retroverted uterus facing the back and have uterine fibroid (it's made me so sad) i'm on CD 12 with a 25 day cycle but af only lasts from 4-5 days... is it regular or irregular? I left the doctor office with no answers because it wasn't my doctors office. I had to go to a different location due to test having to be done from CD 5-11...