Date of conception guess

Marissa

Hi ladies! I have crazy irregular periods (50 days, 80+ next time, etc) from PCOS, endometriosis, and hypothyroidism. Anyway, I got a positive pregnancy test on the 20th, and am trying to figure out when we conceived. OPK strip was positive the 29th, has sex then and the 2nd of February. But then pregnancy test was negative on Feb 16, positive on the 20th. Thoughts? TIA