I think you may have accidentally selected the sneak skill of your Death Knight and attached it to either the primary or secondary hands of that character. Then, when you clicked the left or right mouse buttons while that character was highlighted, you entered sneak mode. Your characters crouch, and their vitality remains either constant (while stantionary) or dropping (while moving). Just hit the relevant mouse button again with your Death Knight highlighted, and that should relieve the problem. You might also want to make sure you have primary and secondary skills properly assigned.
