The song stuck in your head has a scientific name: INMI — Involuntary Musical Imagery

Scientists at Goldsmiths, University of London found that earworms (INMI) are most likely to be triggered by recent exposure, emotional state, or an unresolved musical phrase. The brain's auditory cortex actually replays music even when you're not listening — the same regions activate as during real listening. The best way to dislodge an earworm: listen to it all the way through.

Source: Beaman & Williams, 'Earworms (stuck song syndrome)', British Journal of Psychology (2010)