Kudos to you for trying to get more information. I definitely think knowing more about Bipolar Disorder will help with your relationship with her. Letting her know that you understand will be a big help. I think it is good advice to try emailing or writing when she is having a rough time. There are some great books out there to help you understand and maybe you can by some for your birth Mom as encouragement to her.
Socializing for some people with Bipolar Disorder is very difficult. When you feel bad you just want to shut the whole world out. Do you know anything about the meds she is on?