Dont understand how im the father very confusing?

ok i slept with so and so girl on may 27th 2011 one time and one time only we went our seperate ways she found out she was pregnant at the doctors office on june 16th she says im the father cause im the only guy shes been wit in almost a year heres the kicker though her lmp started april 29th and lasted till may 4th how in the world with those dates can i be the father