The good news is that you did not actually earn negative medals! This is what is happening:
1) You queue into a game.
2) You either crash or disconnect for some reason, and lose medals on the server side (let's say -20)
3) The client (your device) has not yet visually updated your medal count.
4) You queue into a game and complete the match, earning some medals (let's say +15)
5) The game now visually updates your medal count with the server: -20 +15 = -5. so your recent win looks like a negative medal value.
We're working on a fix in a coming update to make sure medals are updated so that negative results should not appear.