Whom would you believe your girlfriend or your bestfriend?

ok me and my girl ve been in a relationship for almost five years but for the last three years we were in distance relationship due to our studies...now my best friend has heard that she is having an affair with someone else and he told me... i wanted to clear the mess so i called her but her phone was off i tried for one month and finally i could connect to her...she was very cold while talking to me even though she refused all those allegations...so now m hung in between and dont know my next step...please suggest me