My macbook pro retina is suddenly lagging in gaming graphics.

Ive installed aassassins creed 2 from the macstore on my macbook retina 15.4 inch base model a week ago...and ive been playing the game since a week and ive been pretty happy with it for a while since ive been getting 45-50fps with my NVIDIA GT650 1GB.. but recently overnight the game started to lag in graphics and fps it decreased to 10-15fps overbight its pretty much unplayable now.. 😕.. i have no idea what went wrong and i dont know whether its a harware problem or not.. ive also tried SMC resetting according to other discussions but it helped a bit even though the lagging reduced it hasnt gone back to its original potential its still lagging by a bit.. please if anyone could help me id be thankful....😟