I Had Sex One Month Back, He Didnt Inject It

i had sex one month back, he didnt inject it inside.but i doubt if a little bit of sperm may have touched vagina. after 3 days then i got light periord for 2 days almost3 days. its more than a month now i fear being pregnant as my stomach is fatty. i have been sitting and reading all the month. confused no periods coming. please help. am i pregnant? can it b posible with out penis even touching exactly near vagina.