bestfriend is trying to hang with my boyfriend?

i've been dating my boyfriend for 3 years and we're both 19. we've known each other about 5 years. my bestfriend has been on and off with me for about 7 years. lately we've been pretty close and that's good. she has her own boyfriend but they break up all the time and then get back together. my boyfriend, A, and her boyfriend, J, are good friends. they go to the gym daily and workout for like 2 hours or so. i'm totally fine with it, i acctually like seeing him getting in shape! but my friend, K, got a job at the gym that A & J go to. this wouldn't bother me if she didn't randomly quit her other job just for this one. then, A texts me and tells me K invited him to eat with her. am i allowed to be mad? A said no obviously but i am just getting a weird vibe and idk if i should ask K about it or just ignore it. i completely trust my boyfriend, K has never really been his favorite of my friends anyway. what should i do???