Oh littleducks - poor poor you. I know how scary and overwhelming this sort of thing can be having had a fall a few weeks ago which necessitated a late night trip to hospital, so I really feel for you this morning.
Firstly, I echo everything merry says, particularly in reassuring you about how good the cushioning is in your tummy. That amniotic fluid really is amazing stuff (plus your baby's head also had the pelvis protecting it now if your bump is so low), and if the doctor was really worried last night there is no way he would have sent you home. Plus if he said the scan can be today or tomorrow that again says to me that he doesn't think there is any immediate worry. I know that won't be much comfort to you this morning as you'll still be shaken but I do think if it was serious then you would still be in hospital.
With regards to getting your scan today, your doctor last night should have written everything up in your notes and signed it. I would take a look and quote his name to the ultrasound receptionist and the specific details of what happened last night including that he specifically said you needed to get an appt today. If the doctor was a junior doctor, he should have got his decision confirmed and signed off by the senior doctor on duty last night so you may also have that doctor's name in your notes and I would quote that as well.
If you find the receptionist is unhelpful, then you should definitely get your midwife team involved. If at this point you still have no joy then I suggest that you and DH get down to the hospital and stand in front of the receptionist. You will need your DH to be the assertive one for you, as from my own experience I felt quite fragile for a good day or so afterwards and couldn't have been assertive on my own, but if you are both stood there with your notes from last night detailing the need for a scan and you say you won't leave until you get one then you are much harder to fob off. I really hope it doesn't get this far though and that they are able to sort out your scan without you having to press too hard for it.
Good luck, and let us know how you get on.