Possible too much antifreeze, not enough water? On newer model altimas they are real bad about not blowing warm if the mixture is not right. They just blow cool/luke warm air until you drive then melt your face lol. So i guess simple way to check is just drain radiator, fill back up with water. Should be enough antifreeze in the block.

Sometimes air in system too, but this is usally after replacing a radiator or some kinda of work on coolant system.