This is a well known effect, and most clearly happens with hot instant cocoa which has just been mixed. Stir the cocoa while tapping the spoon on the mug, and you'll hear the pitch of the tapped spoon go down. Now start tapping while not stirring; the pitch will gradually go up by an octave or more. But, if you stir again, the pitch will go back down again.
The cause is microscopic bubbles of air, which lower the speed of sound in the cocoa and thus lower the resonant frequency of the mug. Heated tap water is supersaturated with air; add cocoa or sugar and the air will come out of solution into those microscopic bubbles, lowering the resonant frequency. Let the cocoa sit, and the bubbles will rise, leaving the bulk of the mug with fewer bubbles and a higher sound velocity, raising the resonant frequency of the mug. Stir, and you'll bring the bubbles back down, reversing the effect and lowering the pitch.