I hope you've managed to sleep and are feeling better this morning OP.
I've struggled with health anxiety a lot at times, especially in my 20s and early 30s, so I really empathise. It's truly horrible to feel so panicked. Mine definitely peaked again when my son was little, and I can feel it again now my parents are elderly and unwell.
This probably isn't of help right now when your feelings are so intense, but I wanted to share a few things that have helped me to feel much better for the last decade or so.
CBT helped me a lot, as did an amazing mindfulness course (before it was trendy!). If you look up John Kabbat Zinn online, he has some things you can listen to and do at home which I found v helpful.
I've learned that keeping my blood sugar as stable as possible helps me to feel less anxious. It's really difficult, especially when you're exhausted and feeling under the weather and reaching for sugary food is all you want to do. But I know if I have a day when I eat erratically and my blood sugar yo yos around, that I'll feel panicky by the end of it.
I was in a pattern where if I felt any slightly odd physical sensation, I'd freak out about it, which would make lots of other anxiety symptoms start, and I'd start to feel I was dying of something and need medical reassurance. So a moment of dizziness for example, could send me into a spiral for weeks - and once I was worried about it, the symptom would stay, even through it was nothing really. I've very slowly learned to accept that my brain is just oversensitive to fairly normal sensations, and I can usually let them come and go.
The vitamin levels are definitely worth checking, as a B12 deficiency recently made me feel under the weather. And again, once there's something like that, until I know what it is it's easy to start catastrophising again.
I hope there might be a helpful thought somewhere in there OP. And good luck speaking to your GP tomorrow. Sending lots of supportive thoughts your way.