Please tell me if pregnant or not?!?

My sister had sex last Friday 06/24/11 she was so upset about it. Lord forgive her though. They day after she took the Plan B pill. And earlier today she went to pee and she noticed like a light blood in her underwear. We googled and it said implantation but then we looked up side effects of the Plan B pill and it said within a week it makes most women bleed and knocks their cycle off track. It said the bleeding starts off light and then eventually gets lighter. She had her period the week before last so basically 3 weeks. Can someone please tell me do you think she is pregnant or do you think that she is not?