Angell,
I don't have any answers for you (that's between you and your docs), but I do have some info and thoughts.
One possible reason of muscles cramping or pulling is an electrolyte imbalance. That's easily checked out with your internist. Make sure they check magnesium and phosphorus, too, because that's not always in a basic panel.
If your muscles have too much acetylcholine (some muscle relaxers can do that) or not enough, you can have cramping and pulling of muscles. You could be a body builder and still have this problem if you have MG.
There could also be an interaction between any drugs you're taking that can cause it to happen.
Driving in a car uses a lot of large muscle groups, as well as the neck. You could try a U-shaped pillow while in the car to support your head (the head is very heavy!). I used to take road trips, but now I'm lucky to make it 20 minutes to the grocery store and back. It all depends upon how your MG is doing, and it doesn't sound as though yours is doing very well!
As far as muscle relaxants (and acetylcholine) in general goes, here's a brief intro.
http://web.squ.edu.om/med-Lib/MED_CD.../020634r00.HTM
And Flexeril might be okay for some MGers (everyone is different, as are certain types of MG), it doesn't sound good for you. In fact, it sounds completely contraindicated.
http://www.drugs.com/flexeril.html
"You should not take Flexeril if you have recently had a heart attack . . ."
That pretty much says it all. I would be more concerned about the relationship of muscle relaxers and the onset of a heart attack.
I have pulled muscles doing what I thought were fairly simple tasks. What works for me is Tylenol/acetaminophen, an electric or microwavable heating pad as soon as possible, staying well hydrated, and massaging any cramping or pulled or tight muscle right away (even though digging into them hurts, it works).
If you do anything such as a longer car drive, make sure that you stretch a bit afterwards and then lie down and rest. Are you resting between activities? It really helps.
Sometimes alternative medicine is what works best.
Annie