I would try to change how you think about what happened last night? You didn't say when/how they made the plans to go out? It could be that the mutual friends were just mentioning it and your flatmate invited herself to go along too? It doesn't sound like a "formal" invitation or planned sort of occurrence? There were probably other "mutual friends" who didn't go either? I don't know either if you have been out some of this week at other events (like your chamber group?) and maybe they didn't think of you only because they didn't know/were unsure of your schedule and thought you might be busy. It can be hard when things are arranged on the fly with no great planning to think beyond the people who are actually there. If you were talking to some of the people after chamber, for example, and they suggested going around the corner for a bite/drink before one of them running you home, would you think to call your flatmates and tell them where you were or that you'll be "late" getting home in case they were thinking of you and worried? I know I wouldn't. So much goes on in a face-to-face negotiation that it can be hard to think of things/people "outside" that? They could only "see" the people there/or that they thought about in conjunction with "soccer" and the event itself. So it wasn't something "personal" -- they didn't forget "you" or decide they didn't want to invite "you", you just weren't the bull in the china shop for them this time :-)

I was with a friend last week and her husband called her cell phone and it turned out she had his medicine in her purse but after we went to the event we were going to, we decided to go to dinner but it was me who suggested stopping first to get his medicine to him. You see how "associations" just may not lead to any other person, no matter how important. We were starving! :-) So we didn't think of her poor husband and how he might have been suffering even.

Too, I'd be glad I wasn't invited since I wouldn't have liked it and I would have turned it down. Wait and hope they remember you when they're going somewhere you want to go? If you turn down too many invitations, some might think, "Why bother inviting her, she always says 'no'." and that's not good?