According to The Concise Book of Trigger Points, issues with the Quadratus Lumborum (a deep 'core' muscle responsible for side bending) can lead to symptoms such as:
Holy moly? Seriously?
Actually I can agree with a number of these especially as it's an area I can have trouble with (if I sit too much or neglect my body/ movement).
Using Tennis balls is a great way to help with Trigger points/ low grade muscle spasms. Just ask my clients who have a love/ hate relationship with them ;-)
Check out this video to find out how to release the QL: