I didn't think her response was all that bad...this is one of those situations where you can think the worst or trust the response. Maybe they did plan it on the spur of the moment...sounds like the opportunity just presented itself and they just went with it. The fact that you weren't invited may mean that you are slowly being forgotten due to you missing out on one too many get-togethers. It does happen and maybe they have also been feeling that you don't want to spend time with them anymore. If you want the friendship to continue maybe try to initiate a get together yourself and reach out to them, let them know that you're still interested in keeping in touch?