Am i really the father seems unlikely?

ok i slept with a girl on may 26th/27th she found out she was pregnant june 16th and told me i was the father since i was the only one she has slept with in almost a year heres the thing that blows my mind she told the doc her lmp lasted from april 29th until may 4th so if we didnt do nothing until the 27th how is it possible that im the father