Did my "friend" use me or am I being irrational?

Basically, I have this "best friend" which I made last year, through another girl who used to be my friend. Me and this other girl (let's call her J) fell out - for reasons I refuse to accept her apology. (In a nutshell, making it known to people she worked with I had depression when it was nothing to do with her) since then one of my "best friends" (let's call her K) ended up not really talking to her much last year..

She said she got fed up of J taking advantage of her, her having to pay for her drinks on a night out and basically moving into her own flat. All reasons were understandable and then me and K became very close friends Recently, since I got a job, J and K have become very close again and since then K has hardly wanted to hang around with me, hardly talks to me etc. I keep seeing on snapchat how they are close again, (J coming round her flat with J's cousin on nights out etc. Night out tonight and was randomly there last night too). I can't help but feel angry... If K thought J was using her so much, why she allowing that again?! And I feel like I have been used in a way. Maybe not intentional.

I dunno if I am being irrational or not. At the moment, I feel like cutting K out TBH. I don't like people who go back on their word but I also don't want J knowing my business which I know K would end up saying to her... Would you say my friend used me? Any advice? Am I being irrational? Opinions needed. Cheers.