Don't mean to be insulting, but this has happened to me and then I realize "Oh wait. I offered the draw" . Of course if it's your turn to move then not the case. If you do move though, that's the same as declining.
Check out Help and Support at the bottom of your screen. In the section of the page called Playing Chess there is information about offering/accepting a draw
In one of my games i am told i have a draw offer pending but am unable to accept decline?