Help?

I got my positive opk 15 days ago, so Glow is saying I'm 15dpo. I haven't begun temping yet, but I figure if I'm out this cycle, I can start the next. I have pretty irregular cycles, but I average out at 40 days, which I think would be lower if I hadn't had a super long cycle in the summer. My doctor says to lose weight, and since I've had a child before (she's 10 years old),  she sees no reason why I couldn't have another. So, I guess I just want to know if it's possible that my DPO count is off a little and that's why my period hasn't come yet? If it's anywhere from 24-48 hours after a positive opk that a person ovulates, would I actually be 13dpo?