on BC but took plan b, Could she be pregnant?

I have never taken Plan B, but I hear that it can normally cause some irregular bleeding. If these episodes of bleeding were not timed to her normal period I wouldn't read too much into it. If they were around the time of her expected period, then I would say you are okay. Especially if you are on the pill, because at that point you are pumping a lot of horomones into your body.

If she is bleeding enough to fill a tampon or pad, the chances of her being PG are slim. Also, 4 pregnancy test can't be wrong. They say the day of your period is the first accurate date, but I found that hard to believe. If she is testing negative 1 week or more after her period she should be getting an accurate result. Make sure she is taking them first thing in the morning and using a good brand like First Response.

My assumption is that she is not pregnant, but only a doctor can tell her for sure. If she is concerned about all the bleeding, call the doctor and verify that it is normal.
Good luck!