Sorry, I did miss it. Well, you caught her in the act and she's embarrassed. The content of the message obviously hurt her and she's blaming you, very embarrassed or thinks you were conspiring behind her back. Have you told her you have noticed some distance between you and and wonder if it is cool between you or if she wants to talk about it?
What happens if you extend an invitation to the couple? It sounds like you are waiting for them to ask you out. Perhaps, they feel they are doing all the work. I'm just guessing.
I know you are posting to understand their behaviour, but what is your gut feeling? How do you feel aside from the frustration of not understanding what the hell is going on?