sounds to me like it could be muscle cramp due to a build up of lactic acid, i think the fact that she has not been run for 19 weeks and has had pups during this time, and she is a heavyish built bitch, and who is maybe not as fit as she should be, could all indicate this, especially if she is very lethargic, panting a lot, doesnt have a good appetite, and it hurts her if you touch her muscles,
try massaging her with liniment, very gently, from her back legs first then towards her head,
dont excersise her much just for now, give her a dose or two of electrolite solution, keep her hydrated