I too experience long anxiety attacks. Though sometimes mine are on a daily basis instead of lasting nonstop for days.

I too have been to the ER for unexplained issues. Did an EEG, MRI, bloodwork, all that, came up with zero.

I recently went to the dr and had my blood pressure/heartrate checked; all healthy. I agree with everyone else to rule out "everything else." Though I find it notable that the Xanax had such a beneficial effect on you. I was on Xanax daily for years (I have Generalized Anxiety Disorder, so my anxiety was a 14 on a scale of 1-10).

For me the odd thing about anxiety is how many ways it presents itself. Sometimes my throat closes up/hard to swallow, sometimes my chest gets stiff, feels hard to breathe, and my breath can be shallow. On occasions even my whole body will feel weak/floppy. When my anxiety depletes, the weakness goes away. It can be quite confusing because anxiety can show up in such various ways, so I understand your frustration.

Anxiety can pop up without any obvious correlation to anything. It just "happens." Have you drank a lot of caffeine lately (coffee causes me to get jittery)? Maybe there's been changes in your life/schedule that you've not consciously been aware of?

I hope you get the answers you need.