My girlfriend was having what we thought was her period. it started yesterday and then stopped today. could this be because we used plan b?

Plan b: Side effects from plan b include nausea, abdominal pain, fatigue, headache, dizziness, breast tenderness, vomiting and menstrual changes. It should be taken at least within 72 hours of unprotected sex. It is 89% protective against an unwanted pregnancy. She should get a pregnancy test if there is any doubt about her being pregnant.

Yes, it might be due: to Plan B. Potential side effects w emergency contraception include menstrual flow changes (spotting, bleeding, early or late period).