Menstrual Bleeding after 15 days of last

Hi My girlfriend had her last period onset on 11th August and it was over by 14th August. Today, on 26th August, she is experiencing menstrual bleeding again along with usual menstrual cramps. Shall we be worried? Here is what happened in the last 15 days between 11th and 26th: 1) Between 14th and 19th, we had sex several times. But I never ejaculated inside her. 2) She had taken an After-morning contraceptive pill on 19th. 3) She have had NO other irregular symptoms in the meantime. 4) She caught cold on 18th, but no fever. Only cough and running nose. Before we start consulting any Gynaecologist, I would like to take advises on why this might be happening. Is it a one time anomaly or is there anything here that might indicate this sudden Menstrual period. Appreciate any information regarding this. Warm Regards, Nihar