Help...vets cant help us...

I have a German Shorthair, Lucy, she is 6 years old and is very high strung, and she loves to run at warp speed along our propery fence line (2.5 acres), she'll run after trucks, cars, rabbits, birds, you get the picture... she over does it, and then cant walk! Her front legs cramp up, first it was her left side, now her right. Normally after she sleeps it off, she's better, with just a slight limp, but this time it's worse. I took her to the vet yesterday because she was really bad. She was whimpering nonstop. She couldn't even sit up. The xrays showed nothing! The vet says it could be nerve or muscle damage and to keep her confined for 6 weeks. (That will be a challenge) He gave her Metacam and a pain reliever, she is eating and walking with a limp, but is there anything else out there I can do for her? I've heard magnesium is helpful, the vet wouldn't comment, how much should I give her? She weighs 35 lbs. If anyone out there has had simular experiences, please share, she is really having a hard time, it breaks my heart, she's such a sweet little thing....