I had an ultrasound at 6 weeks with this pregnancy and it scared me too because we didn't see much, not even a heartbeat. I also had a missed miscarriage so that made it even more scary. The best advice I can give you is to try to stay busy till your next apt (or till the next ultrasound).
I'm currently 28 weeks now, not seeing a heartbeat at 6 weeks is not un-common. Until you can actually hear the heartbeat I'm sure you'll be a little extra freaked out, but just remember to keep busy!
I have seen many posts on here with this problem and it seems like a day or two even a week can make a HUGE difference. Try to relax! I know it is difficult to do, i have also suffered a miscarriage then in my second pregnancy I delivered a healthy baby girl! Have they booked you in for a repeat u/s? Good Luck!
It could have implanted a little later which would be the reason why you did not see much - but try not to worry - it is still early!