No. You need to relax. Her body is going to be kind of weird for a few months now. The Plan B is a mega dose of hormones and now things will be funky for her.
More likely than not, she isn't pregnant. I hope both of you use this as a big wake up call that all sex should be done protectively because no sex is worth weeks and weeks of worry and fear.
When is her period due? Have her take a pregnancy test then. If it is negative (and it will be), invest in a mondo box of condoms! She can also get birth control pills at planned parenthood really cheaply or possibly even free.