Yes, it can be both, but I'll attempt to give you the breakdown here. Keep in mind that neither condition can be measured in exact terms.

Bipolar is a mood condition that, in general, takes time to move from mood to mood, usually over the course of several days, and the mood can last for weeks, months. Even in fast cycling bipolar, people generally don't change mood more than once a day and the onset of the new mood is frequently after sleep with a break in between. For slower cycling bipolar, people can feel the mood coming on...of course, you have to know what to look for and some people can't. This is the big catch with bipolar, however...there is no real cause. The mood comes and goes and changes but not necessarily for a concreate reason. Yes, bipolar can be affected, sometimes greatly by what's going on, circumstances, and drugs and alcohol, but they don't cause it.

BPD, on the other hand, always has a reason. People don't always know what the reason is, but there is always one present. And it has the ability to cycle very, very fast from one extreme emotion to another. This doesn't happen very often, thank God, but it does happen....example. Your SO comes in looking all disgruntled and upset. You AUTOMATICALLY feel they are mad at you and get angry and defensive. They notice your anger and look confused and you are AUTOMATICALLY sad and remorseful that you felt that way. The radio is playing in the background and you hear a song you like. You are AUTOMATICALLY happy and want to dance, but the person doesn't want to....you are AUTOMATICALLY upset and feel rejected. Then your eyes stray to a picture of your daughter who's moved away from home. You are AUTOMATICALLY missing her and want to call... Yes, all of those emotions, extreme emotions, in a matter of five minutes or so.

The biggest difference between bipolar and BPD is that bipolar can often be treated successfully with meds. It is a chemical imbalance or something of that nature, and even though it is sometimes trial and error to find out what will work, many people find close to full relief on the right meds. BPD, on the other hand, cannot be successfully treated with meds. Yes, some people take them, but it is to relieve the symptoms, usually the depression or anxiety that accompanies the BPD. It doesn't affect the BPD itself.

I am BPD. I know what I know of bipolar from reading and from a few friends who have it so my knowledge isn't as good by far, but I hope this helps a bit.