alt+tab then song gets cut

I press alt+tab to switch from one window to another, neither being iTunes, and the song playing at the time just ends. The options and time are the same, but the song just ends at that new time now, nothing wrong with the file either. So far, the only way to fix it is to delete the song and add it again. I did that for the first song that got cut, then it happened again, and I'd rather not have to do that. More importantly, I need to know how to STOP THIS FROM HAPPENING.