What should I do?

There is this guy that goes to my school, we’ll call him “K’. He is dating “J” and they have been together since..October? Well, K and I have always been /friends/ and flirt here and there, but it has gotten a little bit more serious. He plays around, saying things that he never has before, that I know J would kill him for if she found out. He also says things such as “Oh well, when I’m single we can do whatever we want.” Now I have started to actually have feelings for him, but I don’t know if it is mutual. However, during school hours, when he isn’t with J, he is always flirting with me. Just little things, like having his arms around my waist, or over my shoulders, and grabbing my sides. I don’t want J to find out, but I don’t want K to cheat on her, and I also don’t want to be the reason for a potential break up.

I like K, but how do I keep that to myself? Of course I wouldn’t mind all of this to actually grow into a relationship, but there is still J, and she has never done anything wrong to me. Now if you think I’m a terrible person for letting things go even this far, that’s fine, because I don’t care about your opinion of me. I am here for advise on what to say and do.