Thanks, Kim, for giving us a name to call you!
Well, pain meds can definitely make MG worse. Here's how you explain the pain. Pain is not a PRIMARY symptom of MG but it is a SECONDARY symptom. When muscles get weak, they can cramp and spasm, just like they do in any athlete. In people with MG, they just do it a lot sooner. DUH!!!

I've had my entire trunk go into spasm, which is a huge warning sign that your breathing muscles are about to tank. That was right before my myasthenic crisis.
If they don't do the right tests, they won't know how bad you are. They have to do breathing tests, arterial blood gases, a clinical exam, etc. They can't go by how you look or oxygen saturation while sitting still on a gurney! Oxygen saturation tanks late in the game with MG. They need to see how your O2 saturation is while you are walking.
When you rest and take Mestinon and you are not getting better, that's a big warning sign too. Also if you get a sudden shortness of breath episode where it feels like you can't breathe. Definitely time to go to the ER if that is happening. If you can afford one, an oximeter is a great purchase. I've had my Nonin for five years and it's still working well, even after dropping it several times. Nonin has a less expensive version now called GO2. I liken getting Nonin to deciding on a Honda over a less reliable car. You may pay a little more up front but it pays off in the long run. That way you can track what your "normal" O2 saturation is and what it's like when you get worse.
Have you checked your pulse at all? When MG gets worse and you can't breathe well, the heart first tries to make up for the weak breathing muscles by pumping faster - which I'm sure you know from working in the medical field. After that isn't sufficient, that's when the O2 saturation drops.
Did they do the antibody tests while you were on Pred? That can make them negative when they are really positive. Did they do the MuSK too?
Are you home alone? If so, you really need to be careful. Some people actually pre-record a message to play to 911 in case they can't talk!
I hope you will get better. Maybe do a new post asking for good neuros or an MG expert in your area. It really helps to have a neuro who knows what they are doing.
Take it easy!
Annie