Like someone else already mentioned it might indeed be dust buildup.
I have a Dell M4300 myself and it collects a lot of dust blocking the heat exchange causing the fan to come on really often and performance of the laptop is degrading.
Dell has lot of information online that might help you including manuals on how to take your laptop apart to reach the heat exchange.
Give me a call if you need help searching for the information. I actually need to clean out my laptop again as well.