Please help? My period is late?

my period usually comes on the 15-20th of a month but it's late by 10 days now...i don't know what's going on...I've never had sex before so i don't think i'm pregnant or anything o_o i'm a little overweight and my sleeping habits have completely changed since summer break started, but is this normal??? i saw people worried that 3 DAYS is unusual, so now i'm really worried about my period being 10 days late...if there IS something wrong then could you tell me waht i should do??