Someone please help...?

Ok here's the deal it's been 3 years and 8 months since we've been TTC. I'm been to the doctor many times and the problem was I wasn't ovulating so I was taking Clomid. My last dose was 150mg and it stopped working so fo the last two months i've been on letrozole. so here's the story for the last two months my frickin AF has been coming on day 31 on day 21 I get my blood test taken to see if i'm ovulating and I have been. When would I be ovulating, that I can't figure out?? Yes I have been using Ovulation testers I bought a fertility monitor and it never once showed a HIGH reading and I was ovulating....it showed a LOW and a MEDIUM reading but never a HIGH.. Please tell me when I ovulate if I get the blood test on day 21 and it showed i'm ovulating when am I??? I'm so confused....Thank you