Confusion?!

Bex
Hiya, I had my BFP today! But ive only just come off the mirena coil on the 17th of august and havent had a period, but glow nurture asks for my last one. I know I ovulated around the 29-31st, not 100% which as had 3 positive opk in a row. Im not sure what to put as my last AF date until I get confirmation from the doctor. Any ideas ladies?