Who should I reAlly belive???:(?

Ok well I have been dating this boy ,and well I was txt my best friend one day and she said "you know jessy is dating Jerome (my bf) right?"and I was like what?!But..she sais she has knew for like 2weeks,,why didn't she tell me?:(and jeromes phone is dissconnected so idk what to do??any ideas from what I should reaaly belive??:(