Possible Pregnancy questions on birth control.

Hi,

My girlfriend has been using Nuvaring since before we met. During sex it sometimes slips out but she puts it back during the 3 hour period. She got her period during the fist week of March. The last day she did not get it was Sunday 6th. I came inside of her twice that night. On Thursday night we had sex and I came inside of her once. The Nuvaring fell out and neither one of us noticed till the next morning. She got her period again on Sunday 13th. She went to her doctor on Tuesday and she gave her a birth control shot changing her birth control to that now. She also gave her Plan B, just in case. We haven't had sex since that Thursday night.

My questions are the following:

1. Did she get her period on Sunday 13th because her birth control got messed up, or is it possible that it was implantation bleeding?

2. Is there a chance she can be pregnant?

3. How effective would Plan B be after that period of time elapsed?

4. How long before the birth control shot starts working?