Very strange, on the phone try deleting the bluetooth connection (settings, bluetooth, press little i symbol at the far right hand side, forget this device) and then hold the power button at the same time as the home button for as long as it takes for the phone to turn off then for the apple logo to re-appear (this is a hard reset), should only be a couple of seconds. Once the phone has rebooted reinstall the connection to your bluetooth device in case it was corrupted.