My macbook air is suddenly running incredibly slow and lagging, meanwhile the fan turns on when I start it up and stays on full until I shut it down.. What could be the issue?

I opened up my Macbook Air and my fan started running on high and stayed on... meanwhile it was lagging so bad all of a sudden that I can barely use it.. the mouse jumps and my programs won't open, like its backed up. I cleaned up my memory recently so I know it isn't backed up. The fan stays on high from the moment I open it to whenever I shut it down. It has never done this before, what could be the problem???