And I correct on saying that you have a mac laptop?
They are notorious for getting very hot.
I have a cooling pad for mine, although my newest laptop doesn't seem to have the same problems.
Actually now, it's a Lenovo / PC (Windows) not a MAC. But I think when I have it on my lap with a blanket, it gets like a firecracker! Probably on it too long as well!
When you sit with a blanket on your lap, is the blanket blocking any of the air vents?