Hi, well firstly you really need to go to your appointment. You are not going to find answers from non doctors on the Internet. Not to mention these appointments are like gold dust, people desperately waiting for one and if someone doesn't turn up to a booked appointment that's a wasted one that someone else in need could have had.

There are many reasons but the symptoms you are having and majority are not something serious. I had an appointment for a similar reason recently and my husband came with me, there was no issue with that. There is no overnight stay, you get your bloods, scan and a chat with doctor then you go home. There wouldn't be a reason for you to ever stay overnight as results etc just are not that quick. 

I can appreciate anxiety, I suffer myself with horrific health anxiety. But if you are posting on here you clearly are looking for answers and to get those answers you really do need to see your GP who will need to refer you back to the hospital again. Good luck