Your chances at very slim.
You were on your period and took plan b (within 5 hours-- good job!), you're mostlikely not pregnant.
Plan b is a high dosage of hormones and can reallly mess with your mestrual cycle. You may experience some spotting or irregular bleeding, and your period may be heavier, lighter, early, or late for the next 2-3 months.
Try to relax, you're probably not pregnant.