How many days after sex did you get a positive?

I am 6 months pp with no period, so I have no idea if or when I'm ovulating. I had sex 18 days ago. I feel extremely pregnant but took a test a few days ago and it was negative. With my three kids, I found out at 8 or 9dpo each time. So even if I didn't conceive until 4 days after I had sex, that would make me 14 dpo now and I should know by now, right? Dis anyone else get a positive super early with one kid but not u too much later with another kid?