IPhone 4s music notification issue

My iPhone 4s has this problem where I'll be listening to music with the screen locked and I then change the volume using the volume keys on the side of the phone. The phone will then start constantly bringing up the volume notifcation over and over again. The notification will come up like it normally would, then the screen goes blank like it normally is when locked and it'll light back up again the volume notification. It only happens every so often but it's quite annoying having to manually restard the phone everytime this occurs. Any suggestions??