Help!! I Cant pinpoint my Ovulation day!!

Suz

I came off the pill July 17, have used Ovulation strips but I never get a DARK DARK line. I have had a clear/noticable line but its not the same darkness or darker than the test line. I have been getting periods so I know that I am ovulating, but could this be a sign of infertility? I had endometriosis for 8 years, had 4 operations for it, 2 seperate courses of chemically induced menopause so I am concerned about my feritility xx