Oh Lindsay, I'm sorry you're having such a tough time!
Are you having any symptoms that would indicate problems with your liver and bowels? An ER won't just give you a CT scan if you ask for one. You need to have legitimate and/or life-or-death reasons. I remember, back when I was running to the ER for things, there was a woman there who (sounded like) she had very legitimate concerns over her appendix, but the doctor was even debating with her whether or not she should get a scan. They're not going to have you get blasted with radiation unnecessarily.
Wait until your GP appointment. Tell him what's going on with your anxiety. Even if he's not a therapist, he can help you get started by referring you to one or prescribing some kind of short-term medication that can help calm you down a bit. My GP was the one who prescribed me Xanax, and it scared the crap out of me to take it at first, but I'm glad I did, because it did calm me down and helped me see that what I was dealing with was anxiety-based and not some organic disorder.
Also, I think you've mentioned the fact that no one knows about your anxiety—do you think there's someone you can tell or talk to about it? If you're going to treat this thing, you're going to need support beyond that of what this web board can give you.
Again, I'm sorry you're dealing with this. Stay strong—this will eventually get better.