Ovulation question

So, I know for sure that I am pregnant, but I'm wondering which day I ovulated, so I can pinpoint my due date. I was using the Clear Blue digital opk, and it says it's supposed to be positive two days in a row. Well, I started using the test on August 2nd, which was cycle day 10 for me. I didn't get a positive until August 12th, cycle day 20. (I'm breastfeeding my 14 month old so I ovulated late.) So, I only got the one positive on CD 20, and the next day it was negative. Do you think I ovulated on August 11, 12, or 13? (11th - neg, 12th - pos, 13th, neg.) I took them at the same time each day. For what it's worth, we had sex at 12:45am on "Monday night" August 10th, but it was technically Tuesday the 11th. 

Additionally, I had negative pregnancy tests on the 19th and 21st, and positives on the 22nd, 23rd, 24th, and 25th.