Ok i really dont believe im the father how do i tell her serious please help?

ok my last post i posted i slept with a girl on may 27th she found out she was pregnant june 16th we went our seperate ways then she told me she was pregnant that i was the only guy she has been with in almost a year and that i was the father but heres the kicker her lmp started april 29th and ended may 4th so i trully dont believe the baby is mine so what would should i tell her were not together it was just a one night stand