How Is It Possible To Stop Periods After Sex?

hi im 16 and i was wondering if its possible to stop having your period after sex and not b pregnant i have had the usual boating and symptoms but my period is so light that i dnt need anything i dnt need a taampon and such it has been this way for five months about and i have had no symptoms of pregnancy and im a very tiny and small waisted person and i have stayed this way my tummy hasnt grown bigger watsoever so im worried is there something wrong am i having a miscarrige umm please help