Is there a hit point glitch for Noel in Final Fantasy XIII-2?
I believe I just ran into this issue and its solution in my play-through.
You likely have Bloody Medallion equipped. This item will lower your health dramatically.
Here is the description of its ability: Exsanguination: Inflicts significant wound damage to self at the start of battle.
If you use the Optimize option in the equipment menu and have one or two of these items in your inventory, it will likely equip them. You can manually remove the item to fix this issue, and delete it from your inventory or manually set your equipment to prevent it from happening again.