why can't I get pregnant ?

Why can't I get pregnant ? I have four kids already never any miscarriages and current boyfriend I have been with for two years and the first six months we were together we had sex about 3-5 times a day never used a condom or pull out method and I still have not gotten pregnant my periods only last about 2 days when I have one but it's rite on time for the most part and always comes I only bleed for two days could this have something to do with it ?