How much CP do you have?
Each point of CP you have spent adds a little to your stats. With no CP campaigns the extra stats from CP don't count since CP doesn't count and currently all campaigns are no CP.
You're losing the bonuses from CP when you enter cyrodiil. That's where the extra missing health is. You are still getting the 5k from battle spirit.
My DK was sitting at 24-25k with cp in pvp. But, I'm sitting at 22k with no cp week.
You need more health anyways. I don't know your build, but 15k in pve is LOW.
Olava Stone Fist Nord Templar (EP) Pepper Moonchild Dunmer MagDK (AD) Rincewind Redguard Stam Sorc (AD) General Jar Jar Binks Altmer Sorc (AD) Lord Vetinari Stamblade (DC)
Stupid men are often capable of things the clever would not dare to contemplate...
Terry Pratchett, Feet of Clay