Patience is a virtue

I'm not sure if this title fits but it sounded good to me.
I am a little frustrated because I called my OBGYN today because I wanted to know if I have to do anything special for the appointment tomorrow. I'm not sure what to expect at a prenatal appointment. But I did expect to have an ultrasound. David and I and everyone else who I've told are looking forward to hearing what we find out. But..... when I talked to the receptionist at the OBGYN today, she said that it is up to the doctor to decide if we can have an ultrasound. I didn't think it worked like that. I thought that we could request an ultrasound. Also, she (doctor's office) said that I need to bring in a copy of the first ultrasound report that I had back in February. So I called my other doctor and had them fax this report to the OBGYN for tomorrow's appointment.
So basically there is no guarantee that we can have an ultrasound tomorrow. I got my hopes up for finding out if we're having twins, and a more exact due date and how far along I really am. I'm confused about how far along I am. I signed up for some baby/pregnancy websites and they have a due date calculator. When I put my last official menstrual period into the calculator it says I'm 18 weeks. Whoa! Our doctor who I went to in February wasn't too sure either. He thought maybe I'm 13-14 weeks now, but no one knows for sure. I'm really struggling with this waiting and not knowing stuff. I have a lot of unanswered questions floating around in my head and in my abdomen.