I know how exhilirating it is when you meet someone and feel that kind of chemistry.
This girl, K, is in a relationship and the truth is, when a 3rd party invades their space, the two will align themselves against you. You are in a vulnerable position and I don't want you to get hurt. I recommend cooling things off with K, and if she ever breaks up with J, then proceed with pursuing her. If it is meant to be, it will be. I promise.
I know how hard it is to resist someone when you are infatuated, and you may choose to sleep with her, despite the boyfriend. Keep your head on straight. Don't be someone's "side piece". Enjoy the sex but maintain control over yourself and your emotions.
Keep in touch