I really dont think i can be the father?

ok i slept with a girl on may 26th/27th she found out she was pregnant june 16th but heres the the thing she said her last lmp started april 29th and lasted till may 4th if we didnt do nothing until the 27th how can it possibly be my child i refuse to rasie another mans baby we only slept together once to she says its mine cause im the only guy shes ever been wit