That sounds pretty normal really, especially for someone with very low self esteem. Do you have periods that you would define as "highs" where you are very euphoric for no reason and then hit rock bottom? Bipolar disorder has a wide array of symptoms, I was diagnosed with it and have type 2. If you think you have it I would really try talking to a doctor, don't go to him and say you think you're bipolar but make a list of emotional stuff you suffer from, scenarios, etc, be it depression, spontaneous behavior, etc and share it with him or her.
But no, going by just what you've described I don't think you're bi-polar. I'm not saying you don't have it, I'm just going with the info you've provided.