Hi My Gf Had Intercourse With Someone On March 10th

Hi my gf had intercourse with someone on march 10th and had her period on the normal day 12 days later. He pulled out but her period lasted 5 days but its normally 3 days. This month she is 4 days late for her period she took a test and its negative but my question...is there any possibility that its his? If she is pregnant or would it be mine from this month?