I am not sure why my best friend wont trust me?

one of my best friends has kinda got a secret boyfriend that nobody really likes. she has admitted to most of my other friends she has got him as a boyfriend, but in person she hasnt told me. the only reason i know about any of this is because the friends she told had broke thier promise and told me. they also said to me that she specifically said to them not to tell me. i dont understand why she would do this to me as she is one of the only friends i can trust. should i ditch her or something ? please, i need to know what u would do!