Well I think there could be a couple reasons:
1)Maybe Meg secretly has a crush on you and has told your girlfriend, and everytime Meg see's you guys together it makes her jelous. So your girlfriend would think it's weird chilling with Meg with you guys as a couple.
2)Meg could be making fun of the way you guys act together. I don't know if your all "smoochy" but she could think it's funny how you act and shes told your girlfreidn so many times that it's annoying her
3)Maybe they got into an argument and she doesn't want to hang out with Meg anymore. She could have said something.
Now, there are many other possibilities, but those are the first things that came to my mind.