I imagine you posted this with the best of intentions, but it is not wise to diagnose others here or even suggest it because it could lead to inaccurate self-diagnosis. Also, it would be better to say "You may have..." instead of you "You may BE...", because that defines the entire person as their illness. If anything, rapid mood swings over the day sounds more like Borderline Personality Disorder (Bipolar mood swings tend to last longer).

Rapid mood swings can happen for several reasons, sometimes even due to physical issues. I would encourage you to get your blood tested and a physical just to make sure things are physically OK, then talk about potential mental health issues. Regardless of the cause, constantly changing moods sounds exhausting and I hope you are able to find relief soon.