Answer 1

Resetting any language reduces all your xp's in that language & over all

When you play whole load of challenges. When you lose with bronze or silver level challengers.. you lose more xp's [only if you have more xp's than opponent has]

Edit:
upvotes/downvotes, does not make any difference in your xp's. you only get few number of xp's if your answer is "marked as best".

Answer 2

As far as I know, there are only 2 reasons because of which people loose XPs

1. Resting any course's progress
2. Loosing challanges

if you did neither of those then it must most probably be a bug.

Answer 3

I think one your best answer mark or that thread was removed.
We will get (10 + upvotes)xp if our answer was marked as best
Check it out.