I get very depressed easily, but then get really happy, and really depressed and irritable within seconds over nothing. Could I be bipolar?

Unlikely. Bipolar disorder means longer periods of extreme moods — weeks, not seconds, minutes, or hours. Regular (unipolar) depression is more common, & often includes irritability. Brief mood swings are often a personality issue, not a mental disorder, especially if it's a long-term pattern. Some medical conditions & drugs (recreational or prescribed) can make rapid mood swings more likely too.
Mood changes. The mood fluctuation, irritability, aggravation, been snappy zippy and irritability can be part of depression as well as bipolar or anxiety underlying issues need to be evaluated carefully by a psychiatrist before a diagnosis is given.If your functional ability is compromised seeking good timely care will be best decision.We have a lot better meds if needed to address the mental health issues now.