There is more to being bipolar than mood swings, like my current psychiatrist likes to point out every time i see him. However, with the mood swings a few things are necessary for it to be bipolar:

Hypomania (or mania- more severe) which is feeling really really good almost like nothing can stop you, increased energy, decreased need for sleep, and for me I laugh and joke around so much more (almost everything for me is funny!)

Normal - just that... a normal mood state (which I have not really been able to tell if I'm in a normal mood state or not, what defines normal? )

Depression - different levels of depression are present in everyone, but typically there are thoughts of suicide, uncontrollable emotions, and just your typical depression related thoughts (worthlessness, etc..), also increased need for sleep and/or constant fatigue.

What you are describing does not sound much like bipolar, switching from nice to mean sounds more like a personality disorder to me. But then again, I am not a doctor nor do I know exactly what you are going through or how you feel. This is just based on my own personal research into Bipolar Disorder.