Iphone video-audio from drum cover not in sync with original track I heard through headphones while playing?!

I Shot a video with my iPhone 5 to record a drum cover. But when I import the video (my drum audio included ) into iMovie and than import the exact same song I heard through my headphones while drumming over it, the drum and song-audio only remain in sync for the 1st minute but from than on they get out of sync, although I'm sure I played exactly in sync with the audio track I heard through my headphones.

Anyone knows why this is happening and how to solve this ?

Thanks in Advance!