One problem is because of my skinny-ness I often get cramps when walking long distances for a period of time, which really stinks when I go hiking 11 miles in a day.
i got a bad cramp once on a summit, mid way through a 22 mile hike.
found 'gookinade' aka 'hydralyte' and now aka 'vitalyte'. you might try that or another electrolyte replacement product.
perhaps you were too hydrated with h2o which diluted your lytes?
i don't think skinny has to do with it.
and cramps are a real drag.