the display of HP left has been changed. What it now shows is what is left of what you started with on that battle. Not your total. So, go kill a 10 point monster with 10% of your total, get a perfect victory, and it says: 100% left. Then die against a 20 point monster.

This occurred in the last patch. So, basically, you lost. I can't be completely certain of course, you could also have a bug, but if you have screenshots, look closely at your actual health bars, not the xxx% messages, and see.