BPD and bipolar II with ultradian cycling have many similarities, including rapid mood shifts, depression, and irritability. The main differences between them that I've heard of are that BPD mood swings are triggered, while bipolar mood swings can happen for no reason, and that bipolar mood swings include euphoria, while BPD people cycle through bad moods only.

This is the most useful article I've seen on the topic: http://www.psycheducation.org/depression/borderline.htm

I also live in Madison normally; I'm a UW student. This semester, however, I'm studying abroad in Italy.