Is It Possible For My Girlfriend To Be Pregnant Despite Two Negative Home Pregnancy Tests After Unprotected Sex?

Hi, My girlfriend had a condition that she missed her last periods. Since its a week she only got cramps and nothing more . we had an unprotected sex for february 28 and its march 11 now. we have taken a kit test at home on march 9 on 2 test devices , both shows negetive. What we do now?