Most likely overheating. Make sure the thing is cleaned up with some air-in-a-can and that the vent ports are not obstructed during operation. A laptop really should not be on your lap/pillow/blanket or anything else that can impede proper ventilation.
If still not ok, you will need to oopen the case up and do a more thorough cleaning inside, prolly a hairball will fall out of it.
Oh, and it is possible you might have picked-up some malware from all that online movie watching... Run a good scan.