Hello Rox-La-Dragona.
Painkillers function in a way that they add temporary HP that equals the amount player character is missing from max HP, and then it slowly wears off till there is no temporary HP anymore (can even result in event we call "painkillers overdose" when normal health is reduced while on painkillers effect and temporary HP fades away).

From the report we have no way of knowing of what happened there. video would be helpful in this case, without it we can't really identify what happened.