Ovulation predictor test.... Postive on the wrong day?

[dreaming174]

So I have spent the last few months believing I ovulated on day 13, and my gynaecologist said the same. When I had a transvaginal scan on my last cycle, on day 12 my egg measures 1.4, which is a bit small. I suspected my ovulation date might be a bit later.
Anyway, tested last few days, negative until today-day 15. Could any experts tell me when I should DTD?! I tried to upload a pic but phones being dodgy!

I used clear blue ovulation tests. When you lift the test out you'll see a line which is a control and when it detects your LH hormone you'll start to see another line appear which will be faint at first. I noticed this and showed my husband so suggested that we DTD just in case and hey presto the next day it came back as peak fertility when I tested. We did DTD again just to make sure. 11 days later i got a BFP on an early detection test. I've heard that some eggs only last for 12 hours so I'm wondering if this is the case for me and the early DTD helped if I ovulated that day.