My period is very late,had unprotected day 11 of cycle,experiencing lower belly pain&back (maybe pelvis) when i sit down (put pressure on my anus)?

Some possibilities: Pregnancy is the most common cause of a missed period. If not pregnant, other causes of a late or missed period include: excessive weight loss or weight gain, increased exercising, emotional stress, having an illness, traveling, medicines (especially birth control pills), hormonal disorders, etc… A home pregnancy test is very accurate by 3 weeks after sex. Tests are inexpensive; sold everywhere.