That is just how the system works, you -did- actually lose 16 000 XP, however, because you lost a level, it will say it took more XP than you actually lost, because it puts you on the exact amount of XP you'd be on, at the start of your previous level on the character sheet. Since XP progression between levels is hidden.
In this case, I would assume you lost 2 levels. Since 13-14 is 13 000 XP and 12 - 13 is 12 000 XP, the system will then show you lost 25 000. If you try resting, you'll probably get a message hinting you're somewhere in the middle of your level.