Starting at Character Level 6, you will have a Near Death Trauma that will temporarily prevent you from leveling up.
Your blue XP bar will be converted to Red, meaning that you cannot get your blue XP back until you have decreased the red xp bar all the way to the left.
All of the XP you gain while working off the red bar will be converted to blue upon completion.
Near Death Trauma stacks with multiple successive deaths; there are 8 hotkey "bars" of xp in 1 level, each Near Death Trauma incurs 2 bars of red XP. Example, if you die three times, you will have lost 30% blue xp, and then have 6 bars of red XP.