What’s the latest a day-5 embryo can implant??

I’ve been on a total roller coaster since my transfer on April 24. These have been my hcg beta results; I was on the hcg booster from the day after transfer through 5/5 and was told that could account for a number as high as 30, so I didn’t really get a clear “confirmation” until 5/10d

5/3 (9dp5dt): 13

5/5: 15

5/8: 26

5/10: 66

5/12: 131

5/16: 442

5/23 (today): 2058

The first 2 tests, the nurse wasn’t super optimistic-sounding, but said there was still a chance. Since the third test, every time I’ve been told my levels are “rising appropriately”.

I also had an ultrasound today, and they saw a gestational sac in my uterus measuring 5 weeks, 0 days. So, I’m completely confused about what’s going on - did the embryo just implant really late? Is there a chance that this isn’t the embryo, but a natural pregnancy?

Does anyone have any thoughts or insights? Like I said, this has been such an emotional roller coaster, and I’m just over here holding my breath and crossing g all my fingers and toes…