Sight carefully down the neck from the nut end. A lot of guitars can develop a slight hump right where the neck and body join on one or both sides, especially in winter when things dry out. This sounds suspiciously like that. Humidifying for long enought to rehydrate the guitar might take care of things. If it doesn't, the proper fix is to pull all the frets from about the 11th fret on, level the fingerboard, and replade the frets. You might be able to get by having the frets levelled and dressed, if the hump is not too bad. It only takes a little bit of a high fret to cause buzzing, and you generally can't identify a high fret with the naked eye. I use a small straightedge that is just long enough to span 3 frets. The straightedge will rock a little when it straddles a high fret.
Sandy