im playing metro last light and has bad fps

ive been playing it and its been running good this whole time, buy today it runs like straight buns anytime there is combat or something going on. it started running bad getting like 10 fps on chapter "a child" where ur next to the fire stuff and ur holding the dark one. does the game usually run bad during the later parts of the game or is my pc tripping or something? i need help pleasee.