My best friend thinks she owns me?

my best friend is annoyed with me cos apparently i spend too much time with my other friend (who she doesn't like) i dont want to apologise for spending time with a friend! but i dont want this to go on forever, im a teenager so it ight blow but shes being really bitchy about me and my friend, what should i do- its only been goin on one day so far but she isnt the type who just drops things...HELP