Stomach cramps, period only 3days very light, had mornign after pill, negative tests. what could it be?

This may be due to : Your emergency contraception use. Potential side effects w emergency contraception include: tender breasts; changes in menstrual flow (spotting, bleeding, early or late period); loose stools; dizziness; headache; nausea/ vomiting; abdominal discomfort / cramping or feeling tired. If you develop chest tightness, difficulty breathing, swelling of lips, mouth, throat – call 911.