Help! Muscle problems

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From: scott burke 
Email: sburke1@ix.netcom.com

I need some help.  Last week my horse came up lame mysteriously.  She wasn't leg lame it seemed that her muscles were too sore to move.  She had very moderate work on the following weekend only a few hours a day at a walk and trot.  On friday after 3 days of being not ridden at all this problem occured.  She didn't want to walk much and she went off her feed a bit.  The vet came down gave her some muscle relaxent and in a few hours she was fine.  We had some blood work done and it came up negative for tieing up.  The potasium levels were high though which means she could have hypp.  But her symptoms don't seem to match the hypp profile.  She wasn't under any stress when this attack happened and she had plenty of water in front of her.  She's totally fine now so I am at a loss.  Does any one know of any other muscle diseases this could be?

Any help would be appreciated, thanks