What Causes Missed Periods?

hello,
what is the reason for missed periods??
i had my last periods on may 9 and still i havent got my periods..
i had unprotected sex in feb..is it possible that i mite get pregnant now??
i even did a pregnancy test at home but it was negative.
o can it be coz of any other reason??my mom n dad both have diabetes n rite now have become a bit fat..is it because of dat??