it's a glitch. Sometimes after being rewarded tokens from events, it doesn't register for the amount you have but you can still use them so it calculates into the negatives. Restarting usually correct itself. This is an issue that has been around for a long time. It doesn't effect you in any way other than seeing your numbers go negative. Plarium knows this and probably doesn't have it as a priority.

It's a minor visual bug, often due to sync delays. Restarting the game often ccorrects that.