I can relate to the stiff painful shoulders and neck. I had that in spades, but coming and going, only happening when I went into "attack". I discovered that I had Porphyria and I was going spastic when I was exposed to a trigger--a food trigger, medicine trigger, or environmental trigger--and occasionally a trigger such as others describe here, involving the stress of too much exercise or socializing or trying to get to work when you can't make it. I could not take any muscle relaxers, but I learned that the attack would go away if I avoided triggers--but here's the catch, for years I didn't know what all the triggers were.
Now I am not suggesting this is your problem, but of course it could be. That is why I am writing this. You may be reacting to a trigger--even if you do not have the same disease configuration I have. It could be a reaction to exercise, as you have suggested and others confirmed. I have to be very careful how much exercise I get, and space it properly. I must get SOME exercise but must space it. Not like when I was a young girl and did both ballet dancing and running and seemed to be able to go on indefinitely--until my first bad attack at college, which laid me low and made even getting through college iffy, since I was not dx'd. By middle age I had an MS dx, later a Porph dx, but mostly I was on my own, and prayer and distraction were my chief ways to cope...drugs didn't help, psychiatrists didn't help most of the time.
I wish someone could say what triggers MS attacks. I know my own triggers for Porphyria now, but I also know that high fat is a trigger for MS and I must follow the Swank diet. Last night I ate ice cream (full fat, a rare thing for me) and I was awake much of the night with my limbs and lower back jerking and spasming, and I had to take an extra Miripex to even stop it by 4 AM. I feel the high fat was at least part of the problem, with slow progression adding to the problem.
I feel I could not cope without high doses of magnesium to relax the muscles, which a neuro suggested when I could not take Baclofen 25 years ago. 1050 mg a day now. Also B1 and Calcium. I can take nothing for spasticity except supplements and I hope I always have enough money to buy supplements and the gov't allows access to them.