shorter pregnant than expected

I didn't see anything at all, maybe because I'm less than 5 weeks pregnant...?? But I have to go back next Tuesday for another internal ultrasound. The doctor and I thought I was about 7 weeks pregnant, but she said I'm probably only 5 weeks pregnant... I had 4 negative tests, and on Monday I got 2 positive ones. The first one took an hour before it turned positive, and the second one was positive within a second...

For me, it's a wait-and-see thing whether they'll see something on Tuesday when I'm 6 weeks. Shouldn't I see a beating heart then?