iPhone 4S music skips back to previous position when unlocking.

My iPhone 4S plays music perfectly in the music app, any other app, and even when locked. The problem arises when I unlock my phone. Whenever I unlock my phone, the music skips back to the playback time it was when I originally locked it. This happens with every song, every playlist or album and every setting I've tried, which is most. Even when the music has passed on to a different song, the playback just moves to the time of the current song. I have also tried locking it at a position that is greater than the next song, and all it does is skip to the end of that song.

Do you guys have any idea what is causing this and how to fix it?

Thank you very much.