Stuck in the middle of adult friendships

I have 2 good friends, one I'm super close too, let's call her Bec. been friends for decades, our kids all grew up together. She is by far my best mate and have that , go with out seeing each other for months and things just continue to go on as always. I can honestly say, we have never had a fight. I made another friend a couple of years back, she will be Niki, and I introduced them and they hit it off well. I really like this woman, but she's not what I call a best friend, she is just a friend. We all have lunch dates every so often, hang out and have a drink here and there. We are all single mum's and have alot in common. But out of the blue, Bec is showing jealousy iv never seen befor. Niki often has lunch, dinners with her best friend, Lora. They post on face book and you can see them having so much fun. Me, I'm like meh! I'm happy having one friend I'm super close with and not fazed by not being invited, but Bec has really taken it to heart in a manner I find a little strange, it's like she hates Lora for not inviting "us", yes me included. She's become obsessed with being left out and I don't know how to approach it. She's so worked up about it. Bec has several other friends who she will hang out with almost daily, she's never been one to feel that a non invite means not liked. Iv gotten to the point that if I have a random play date with Niki, I have to keep it secret as Bec gets so upset. Niki is beginning to pick up on it all witch has put a bit of a strain on my friendships with them both, as Niki has anxiaty, and is paranoid about people hating her or talking about her. I'm stuck in the middle. I don't know what to do