Took a plan b one step within an hour of sex.. when should i expect bleeding? will this throw periods off?

When to expect : Bleeding varies a lot and yes it can cause hormonal havoc and throw your periods off.

Varies widely: It really depends a lot on where you are in your cycle. However, there is some chance for breakthrough bleeding with the Plan B regardless of your cycle and no real way to predict it or to prevent it. If you do not want to become pregnant, consider effective ongoing contraception.