It was because of my reaction to an antidepressant that my doc first floated the possibility of bipolar. At that point I had never heard of BPII so I was skeptical. However, with the dx in mind I educated myself on symptoms and the light came on. I could look back on my life and identify various episodes of hypomania. ...It was no problem at all finding lengthy periods of deep depression. That said, I don't exhibit "severe shifts in mood" either as you put it, Pacman.
Learn about the nuances of symptomology and then examine your life again. To conclude, use reputable sources for your information, not just popular culture's image of bipolar.