How many weeks before you can tell they're pregnant?? She tied on June 19, and 20, and the vet said I have to wait at least 6 weeks before we would know

It's been about 1 month or 4 weeks, she seems the same

Could it be that she is not pregnant?? I mean she had at least 4 ties in 2 days, that I counted

. I checked her nipples and her belly but it seems normal