That would be the debt bug. All it is, is the exact percent of your current exp (so, if you have 36% exp, you have -36% debt). It's a result of some recent changes to how the game shows debt - Best fix is to turn off the debt percentage.
they fixed that bug i thought to show true percentage, beacuse it worked on my old exp bar after the patch.
True percentage of 0% would equal the opposite of XP so 30% XP = -30% debt. The debt sees the XP as it has to make up that 30% before it calculates the real debt. Just a minor bug it will be fixed soon. All they have to do is set debt = to expierence when debt = 0 -30% + 30% = 0%