You have to clean it, i have never cleaned a Laptop before but i've cleaned my PC and the Heatsink of the CPU is what generally gets full of dust
http/www.tomshardware.com/forum/366972-28-cleaning-laptop-heatsink
This is of course assuming you haven't done this in months, if you have, then it might need a new Thermal paste, if you ALSO already did that it might be a problem