Confused

Nicola • Married. 29 years old. Mmc at 11 weeks July 2015 and mc at 4+3 weeks Nov 2015. Pregnant with baby girl due 30.07.17 ♡♡
I took an opk on Wed and it was positive. I also had a positive on Thursday morning but Thursday evening and today it's negative so I thought I'd ovulated but I had an ultrasound scan today to check for pcos as I haven't had a period since January and the synogroher said I was about to ovulate?! If I was about to ovulate why would my opk be negative? Xxx