What most likely has happened is that you've gotten some moisture of some kind either under the fretboard or in the slits where the frets are installed. That has caused a small amount of swelling and has very slightly pushed those frets out enough to cause them to fret the note a fret higher than you have intended.... It is pretty common and the way to fix it is to either have the frets recrowned to take off the small amount the causes the buzz. However a partial or full refret and fingerboard leveling would be the best thing you can do.
It sounds like bad news... but it's pretty common. Take it into a tech and have them look at it. They'll probably tell you what I just told you but if there are any other issues they may be able to address them as well. They can also point in the direction of where to get the work done if they can't do the work themselves.
Good Luck