I have a 1998 Ford Ranger Extended Cab. The air conditioner was working fine a week ago and then I went to use it yesterday and it blows out hot air. When it's on A/C it always blow out hot air but on max a/c it blows cold (really cold) until the other day. I thought it needed some freon but when I checked it, the gauge read there was plenty of freon and none was needed. What could cause this to happen when it was working a week ago?