I'm 6 weeks pregnant calculated from my lmp and i had an ultrasound today that only showed a gestational sac and that's it. is this normal?

Could be: At exactly 6 weeks you'd expect to see a GS, a yolk sac, and a fetal pole, about 90% chance of seeing heartbeat. BUT that does mean exactly 6 weeks, so if your periods are not 100% regular, or if you are somewhere in the 5th week what you saw may likely be normal. Be patient until the follow-up, things will become clear quickly - and good wishes.