sounds like atypical migraine (hemiplegic type) to me.
There does not need to be a headache.
Barometric changes, such as climbing, are a well-known trigger.


http://www.mayoclinic.org/diseases-conditions/migraine-headache/basics/symptoms/con-20026358

does not sound like AMS, as the arm symptom is not typical, and you got better when you got to the top. With AMS, would have expected you to get progressively worse. For you to have frank neurologic symptoms, with AMS it would be Cerebral Edema, which should have had a very different course.

However, going up with the onset of this is insanity.