Heater keeps going off

[hookedup240]

Hey what's up guys, this morning i am sitting there warming up my car before i go to work. And the car is pretty warm and all of a sudden the defroster stops blowing heat. I turned it off then put the heat on dash and it was on for like 15 seconds then turned off again. It kept going off so i just left it off then. Never happened before to me. Can anyone tell me what's wrong and if it means something. Pls let me know.

Btw i have a 91 with 111,000 miles.

thanks in advanced