Just remember this... relationships are built on a foundation of trust... without trust, there can be no meaningful relationship. So as you chip away at the trust with your worries... ask yourself this, is it wise to do this?
The way your boyfriend is acting suggests that you seem to be rather 'clingy' (or at least perceived that way).... if this is the case, then he feels he needs some space. He's doing you a favor, by giving you a chance to determine if you really trust him or not.
Because he has plans that include you the day after the party... this shows that he does think of you and wants to include you. However, he wants at least one night all to himself.
As for the women... so what? If he's a decent guy.. you have nothing to worry about --- drunk or sober.
Relationships fail all the time... often times for trivial, meaningless things. Don't let this relationship be another to add. If you persist with these insecure thoughts... you will ultimately find what you're looking for --- a reason to end your relationship. And that reason will be your own insecurities.
So relax... and TRUST him.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen