So I have a problem

Yesterday in choir I was talking to my friend K about performances in our music class. The theme for our performances is Broadway, so I told her I was singing Requiem from Dear Evan Hansen. It turns out she and this girl J are also singing that song, so K and I decided to combine groups.

Today in music, K and I went to go practice while we were waiting for J. J wasn't there at the beginning of the period so K didn't get to tell her that I was joining their group. J comes into the practice room and asks if K replaced her, which she hadn't. We started working on harmonies but J seemed mad at K for the whole period.

When J wasn't around, K told me about how J wasn't cooperating with her and how she wasn't willing to listen to her when she was trying to help with her harmonies. J isn't mean as far as I know but she was being really stubborn about it, since K was only trying to help. Apparently the practice that I joined was the only practice that they got stuff done.

K kept telling me that I was saving the group but I can't help but feel like I ruined it since J didn't really seem happy about me being there.

What should I do?