Cycles while Breastfeeding

I'm BF my 11 month old and I've had 4 periods. All 36-39 days in length. I do have 4 children however this is the first I've successfully BF this long. I am currently on CD 41 and my fertile window was between sept 26th and September 29th. I had a positive opk September 26th around noon. Tested this Am with a BFN. I know opk aren't 100%, so if you've BF and cycles returned, have you skipped one after having so many in a row regularly?? I'm grasping at straws here. Hoping for a BFP but I would be happy to see AF at this point just for some mental relief. Thanks!