Friend trouble. Please help.?

Okay so my friend Lindsey and I have been hanging out a lot lately. But her other friend, (who I am also very good friends with just not as close) Katrina, is getting mad. She keeps yelling at Lindsey about how I'm taking her away from her. And tonight she bitched at me about it. Lindsey is allowed to have other friends and I get that she feels left out but she's been a really bad friend to both of us lately. If you try and talk to her about anything, she makes it about herself. Thats not what a good friend does. She's just been so dramatic about this whole thing and now we're in a huge fight. What should I do? I feel bad for getting in the middle but I didnt do it on purpose. And Lindsey has told me that she would rather be friends with her than be as close as they were before. I dont know, Im just confused. Please help.