Passenger side blows heat when AC is on

So past few times I've driven with the AC on, it felt like the van was still very warm inside. I just happened to reach over and feel the passenger vents and they were blowing HOT air like the heat was on. The driver side and rear vents were blowing cold, so the issue is the passenger side. What could cause only the passenger to be heat when the rest is blowing cold. They are all set for AC BTW