The expression "You can't have what's already taken" have never been more true than the moment you're off the market & the I becomes us.
Is it the little smile that lurks in the corner knowing somone is thinking about you that boosts your looks or the fact that human nature encourages you to chase what you can't have?
My limited experience is also rooting for option number three: You simply don't want to hurt the other side of Us and therefore everyone is flirting with you.
Or you were rather dense and sucked at seeing the signs when you were single.
Either you get pretty being with somone, gets IQ-boosts from relationships or your ego implies it's on the rise. No matter which it is, there is always time for a day out drunk with the girls.