PCOS confusion

I was diagnosed with PCOS at 14. My current OBGYN confirmed it in 2015 after I had been off the pill for 3 years and hadn't conceived. (We weren't actively trying, but also not preventing). She prescribed Letrozole in 2015, and on the second month I got pregnant and now have an awesome 2 year old. I breastfed my son until he was 20 months and didn't have a period the entire time. My cycles started again about 2 weeks after he was weaned fully in February. I conceived in April but it resulted in a chemical pregnancy. Haven't used opks until this month. I have an appointment this week with my doctor to discuss using Letrozole again since I am 28 and want to have another soon. How can I go from a clearly negative test one day and the next have a clear positive? Also, glow is apparently off in my predictions which is frustrating.