Hello! My Girlfriend And I Had An Accident The 3

Hello! My girlfriend and I had an accident the 3 days ago in which the condom broke. She took an emergency contraceptive (Plan B) 9 hours after the accident. A day and a half later she said she is having pretty bad lower back pain. I am assuming this is a side effect of Plan B or could she really be experiencing pregnancy side effects just a day and half after the accident?