what the heck happened?

Breezy

So this is worth posting about, imo. I took 3 pregnancy tests, the cheap ones from Amazon, on November 29 all faint positives. I took one on the 30th slightly darker and another on Dec 1 still getting darker. I go on about my business schedule a doctors appointment they pushed the appointment out until December 27th. As someone who has had multiple MCs I decided since I had my rainbow baby back in April 2017 that this time I wasn't going to wait 18 weeks to share the news. I was tired of living in fear. Plus I figure if the unspeakable happens I was still pregnant and news of that baby was still worth sharing. So on December 10th I decided to take another test just so I could post a fresh pee stick picture on my Facebook. I bought an equate early pregnancy test. It turned instantly positive even darker than the previous tests. I see my Dr on the 27th but the US tech was out. I basically walked in the room sat down and my Dr shook my hand and congratulations then told me to come back on the 4th and we'd do an ultrasound. So the 1st day of my lmp was Oct 28th. Negative test on November 23rd. Positives as noted above. Glow dated my conception as November 11th. I did in fact have sex that day. I had classic signs of ovulation that day as well. Today I go to my appointment and the first thing we do is the ultrasound. The tech was STRUGGLING. The Dr was in the hall and heard us talking so he came in and stood there a minute. They were talking to each other. Then the tech says lift up your hips at an angle. I did and the Dr says, right there get that one. Then he starts talking about dates and he's squinting at the screen. He says Breezy we're off on your date by about 3 weeks. I haven't measured yet but I'm going to say 6 weeks and 3 days. He took over on the US and said here she can't show you but I know you need to see this. He turns the screen around and I immediately see a tiny wiggling white spot. He says there's your baby. Heart beat is good 143 bpm and yep you're measuring 6 weeks and 3 days. According to glow now my lmp supposedly was November 27th. I didn't have one and I conceived on December 5th. How is it even possible to get at positive test 5 days past ovulation. Why was my body producing enough hcg at the end of November to show multiple darkening early pregnancy tests? They need a minimum of 20u to produce positive results. Does anyone have a reasonable explanation or similar experience?