Sorry, afraid I can't help you. But I do have the same problem every now and then.
Usually it is "fixed" when I log out. Obviously not permanently since it comes back.
Just going to follow this tread to see if anything useful will come from it.
This happened to me in Cyrodill. My health would only get up to about 75% and no amount of potions or healing or staying out of combat would get it back to 100% It did after I died though so maybe that was something else, Im not sure. Cant seem to find anything on it