max health cap goes down because of injuries, like laceration, infection, broken arm or leg. Just klick on the red icon of the injury (on the left side of the screen) or go into your character screen and you will see a description of the injury and what helps to make it go away.
The medical system has been diversified and upgraded for a while now. You don't always get an option to slap a bandaid on something and call it good like you used to.