The only person who can say whether or not you are bipolar are you and your therapist or psychiatrist. There are clinical tests and guidelines for diagnosis that can help these medical professionals decide if there is a problem.
However, the fact that you are letting this get to you makes me wonder... (if the concept had been utterly silly, you would have just laughed it off)... is it possible that you sense something wrong? (I'm just wondering)
In any event, it might be helpful to see a psychiatrist, if only to be able to tell your MIL that you have seen a professional, and you are perfectly fine! At the very least, do not let her label your baby. Nobody can tell if a child that young is bipolar.
Robin