What Causes Delayed Period?

i n my bf had sex on13th april..actually he just inserted it n den quickly removed it within 3-4 seconds..it happened twice but he quickly removed it in both the cases.his sperm didn't enter inside me,later while we both were standing he threw his sperm on my stomach..but i missed my periods in the month of april..it usually starts at the end of the month..i tested for pregnancy on 1st if may but test came negative..so can i be sure dat i m not pregnanat i m feeling really worried about the missed period thing i m also not feeling any af the pregnancy syptoms??