Something went down tonight, not sure yet what.

Today was the first time the sun was out and I was home at the same time to do any riding.  My daughter recently got a horse so we thought we'd go do some slogging on the trails.  Phebes had a good mile or so warm up before we got to the hills which we always canter.  They are small steep hills, but the duration is only a hundred-two hundred feet before they level again.  We cantered up one little steep hill, then leveled out, went down a little hill, saw there was a wash out, reversed and cantered up that hill and on up to the top the opposite direction.  Then walked down a rather steep, long and muddy hill really slow to the bottom only to find the creek was too flooded to cross, which meant we had to head back up the hill (slowly).  Suddenly Phebes didn't want to move, and I looked back and her hind muscles were quivering on both sides really bad, and she as breathing much harder than she should have been given we were only a mile and half out.  I told my daughter I thought something was wrong, and decided to get her back home as we were in the middle of the woods.  Like to never got her home, she didn't want to walk.  So I'm thinking tie up again.  I hand walked her the rest of the way home, and took a short cut through our woods.  When I got home it looked like she had a muscle spasming near her right stifle.  I put her in her stall for awhile and rubbed at her hind muscles.  She seemed kind of tight.  She stuffed her face full of hay for awhile, and acted like she wanted to take a nap.  Her muscles pretty soon look nice and loose again.   I took her out of her stall and asked Doug to watch while I did a short trot out to see if she was off anywhere, or if she refused to go, or had muscle quivering.  Nothing.    No gait abnormality.  No unwillingness to trot.  No twitching muscles.  What?  WHAT?  I'm just beside myself.  Can a tie up act that way?  Or could she just have had a muscle cramp?  I checked her pulse as soon as I got her home and it was 59 bpm and I only think it was that high because of the 4-wheeler going down the road.  She is out in the yard right now stuffing her face with grass, and when she moves everything looks nice and jiggly, but she was obviously in some distress out there today.  I've never had a horse get a muscle cramp before so I have nothing to go on, and my experience with the tie-up was not at all by the book, even the vets missed it initially.  Her eye is relaxed, she isn't sweating, she drank from her water tank, and is stuffing her little face.  This really scared the crap out of me.  I'm just not sure "what" happened.