Iphone 6: music starts playing by itself

Hi,

It started after update to iOS 8.3. Music on iPhone will start paying by itself when the iPhone is locked. I had it happened multiple times and can not figure it out what's causing it. Last time it was in my pocket at work and all of a sudden it started blaring out of my pocket during the meeting. It was quite embarasing.

Any suggestion how to stop this is greatly appreciated.