Re: Am I being unreasonable?
OK, I'll (sort of) dissent.
If the question is: would it be unreasonable to tell flatmate not to have his girlfriend over because you're having a bad week, then I think that would be a little bit unreasonable. It's his flat, too, after all. (Wishing she wouldn't come over, but not telling him she can't, is a different story and is perfectly reasonable.)
However, if flatmate expects you to have something to do with his GF -- entertain her, make her at home, make conversation, or whatever -- then that's unreasonable on his part, and it's not unreasonable on your part to refuse when you don't feel like it.
So, not wanting to interact with her beyond a polite hello is perfectly reasonable. She's not YOUR GF, after all. If you don't feel like company then they need to go off and do their thing while you do your thing.
That's my $0.02 worth, anyway.