The plan B pill (Levonorgestrel) has been associated with a number of side effects, including: nausea, abdominal pain, fatigue, headache, menstrual changes, dizziness, breast tenderness, vomiting, and diarrhea. However, I could not tell you more without knowing the type of pain (eg. sharp, stabbing, dull, nagging, stinging, prickling, burning, deep, etc.) And exactly where it is located. UPDATE: You could have a muscle strain, asthma, or pericarditis, tell me if you have had asthma in the past or currently have a fever UPDATE: If you do not have Asthma, it could be pericarditis, do you have a fever? Also, tell me any other symptoms and any applicable conditions you have (if you feel comfortable telling me), as this helps in diagnosis. Also, do you play sports that would use the upper body heavilly, as this can cause muscle strain. UPDATE: Have your doctor take a look at it.