I think that your symptoms may very well be due to sun sensitivity secondary to the doxycycline. MS can present in various ways, and early symptoms may include numbness, tingling, loss of balance, blurred vision, motor weakness... I really don't think this symptom you are having is a due to MS (the fact that it is brought on by water touching your skin makes me think it is much more likely be to sensitivity). If it is not resolving over the next week or so, or if you are having other concerning symptoms, follow up with your MD.
Good luck,
JMK MD