Is it strange that I get antisocial when people come to my house?

I seem to need some notice before people pop by so that I can get into a mental frame of mind to chat and play hostess. We've got some friends over now - my flatmates invited them over, but didn't tell me about it until they rang the doorbell. Now they're all being noisy and happy and I've snuck off to be well, by myself.

I'm fine if I've got lots of notice, but intrusion is intrusion even when they're people I know...