Ok i dont think im the father how do i tell her?

ok like i said in my last posts on here that i slept with a girl on may 26th 27th she found out she was pregnant june 16th and told me i was the dad cause im the only one shes been with in almost a year but heres the kicker her last lmp started on april 29th and last till may 4th there is no way im the father but im to scared to tell her how do i tell her