Air conditioning stopped blowing cold air

One day my air conditioning suddenly stopped working and only blowing hot air. If my car is in the shade or it is cold outside it will blow cool air but once the car starts driving that air will go to hot. I thought maybe it was low on coolant or it needed to add Freon. But coolant levels were normal and that would not explain the change overnight. Then I went out and purchased Freon just to have the odometer read error. I really hope this is not an expensive fix. Can someone tell me an easy solution or perhaps a suggestion of what happened?