Is my girlfriend pregnant. To be taken?

Hi Doctor, I had unprotected sex with my girlfriend on 10th February and she took emergency pill (i-pill) same day and she got her periods by 14th February. So we were expecting her next period on 20th of March. However, she didnt get period. Then again we had unprotected sex on 23rd March and again she took emergency pill(i-pill). However, its been more than one month(last period on 14th February) her period is due. We are really terrified. She tested twice with the hcg urine test pregnancy kit but the results were negative both the time. We are really worried whether she is pregnant. Normally she used to get period within 4 days after taking emergency pill. Please help us. Whether she is pregnant? What measures can we take to get sure and future steps to be taken? If we go by last period ie 14th February her periods are overdue by more than 14 days ie as on 29th March. Regards