Recently I took a trip, and when I got on the interstate I noticed that my AC was only coming out of the defrost vents in the front. The lever is on max AC and not on defrost but it only comes out of defrost. Heres where it gets tricky; when I decelerate or brake it comes out of the regular front vents. Any suggestions?

All I've done lately is put in a new battery and changed the oil, after both of that it worked fine until this weekend I noticed it.

Thanks in advance to anyone who has a response.