How to Play videos in music app on iphone ?

Just today morning for the first time to my great surprise a Video Played out from my iPhone music app, What was even great was that there was an option to play thr video in background :O :O
The name of video was what I had in videos app. I don't know how that happened. I have iPhone 6 plus on ios 9, In evening I synced my iPhone with my computer and after that I tried to search for the same video songs in music app but it is not showing there 😟 I have tried a lot, whats strange is that video is still in the videos app. Can anyone tell me how this happened and what to do to play all the videos again from my iPhone music app.
Please help. Thank you