In need of some advice. I've known one of my friends for about 4yrs and about 18mths ago I introduced to her to our toddler group (about 6 of us who meet up wkly with our kids). She has clicked very well with one of the women in the group and they have become very good friends as have their husbands and kids etc.
While I'm pleased she has made such a good friend, the problem I have is that my friend keeps blatantly planning and discussing with the other person all the things they want to do together in front of me, swapping diary dates etc. I find it insensitive/rude that they are discussing their plans like this and not inviting me to any of these things. I wouldn't dream of doing that to anyone. At first I let it slide and didn't think much of it but it has happened the last 4-5 times we've met and now I am upset about it. I know it sounds petty and I shouldn't feel bothered about it but I don't understand why she is being so exclusive about their friendship and almost rubbing my nose in it 
I'm starting to feel like a gooseberry, I don't know how to react and wonder if I should say something to them next time it happens and what would I say? I don't want to upset anyone either, please help