It's probably nerves or gas. If you took the plan b pill within the 72 hours as directed, you should be fine. That pill does a lot of different things to women's bodies. Some have delayed periods, some bleed within hours of taking it and others keep having a regular period.
Worrying and stressing can cause a delay in your period
It's your nerves. There would be no way you'd start to feel anything yet. Some woman don't even know they are pregnant until later then that.