Jesus that's a lot of ultrasounds. I don't think you need to worry so much. I think if there was something weird going on the doctor would have mentioned it at some point. Don't stress... That's bad for the baby.
The baby's heart beat can flunctuate easily whether under stress, from light or the pressure of being touched with the wand.
If you've been for 6 ultrasounds and they said you're only having 1 baby... You're only having one baby.