Thank you! That really puts my mind at ease!
The arthritis medication (probably some variant of Voltaren) causes some imbalance in salt ions (potassium, sodium, etc) similar to what happens after intensive exercise (leg cramps, slight tremor behind knee, and so on); drinking water and resting will fix it in a few minutes. Ask the people again about the name of the arthritis medication to make sure, but I'm fairly certain that's the one, patented by Ciba-Geigy (as it then was) in the 1970s. Very common.