Can anyone help how far along I could be

I haven't had a period as I'm breastfeeding my first baby. I had been tracking with opks to see if my cycle was coming back. I had no lines at all leading up to intercourse. We had sex on monday 30th at night and the next morning I had a second line on an opk (still wasnt positive) and I tested later that day with no lines again. I found out I was pregnant 12 days later. We only did it once when would you say my due date it?