Why does my compressor turn on when i use my heat?

i have a 2000 dodge ram 1500. i just noticed the past few weeks with the cold weather that when i have my heat on, the compressor is turning on and off. there isn't a on for a/c to be on or off. the knob has to be turned onto vent air. but when i have it on defrost, vent, floor, etc the compressor is kicking on and off as if the a/c was on. what can cause this or is this something that needs to be fixed?