If I'm not mistaken Bi-polar disorder is when someone has massive mood swings, namely between extreme depression and extreme hyperactivity/euphoria.
Tsundere refers to a character that is initially cold towards someone then gradually opens up and becomes warmer to that person or, more commonly, someone who acts cold towards someone but actually cares, etc. about them.
So a tsundere wouldn't really be swinging between moods but more masking their feelings with hostility (for the latter).
If I'm wrong, feel free to shout at me.