I’m so confused with today’s ultrasound

So 3 weeks ago when I was suppose to be 7 weeks, I had gone to get an abdominal ultrasound & the tech said it was too small to see & it looks like it measured 4-5 weeks. Fast forward to today, I’m suppose to be 8 weeks according to her measurements last time & again she said it’s too small to see & said it can be 4-6 weeks. But we were able to hear a bit of something & she said it was the heart. But it was for a split second that we heard something. Has anyone dealt with an abdominal ultrasound like this? My appointment for a transvaginal won’t be for another 3 weeks & now I can’t help but worry.