Plan B doesn't cause miscarriages it prevents the egg from implanting if you got pregnant but once the egg implants and you take it, it doesn't do anything. A lot of times with the Plan B pill it will cause you to bleed irregularly in the middle of your cycle. Its completely normal and on the box it should be listed as one of the side effects.
Plan B is meant to be taken within 72 hours of unprotected sex. I have taken it in the past and it never caused my period to come immediately? Were you due for your period?
Is the plan B pill the "morning after" pill??? If it is and you were further into your pregnancy it could be causing you to miscarry....or it could very well be your menstrual period.
Yes, it is normal for that to occur. I would count it as a period if it lasted longer than 1 day. If it did not last longer than a day, then when you expect your period this month you should test to be sure.