'Could be an issue with your mixer doors.... check other threads on this problem.
If you have "dual" climate control, try switching back & forth between common & dual, and between defrost mode, floor mode & A/C mode..... playing around with this 'might' break loose a "stuck door" problem. That has happened to myself & others. The more proper fix of course would be to get in there & fix that malfunctioning mixer door. But doing this "hick-fix" will validate that the door is in fact your problem. (under dash)