The sudden urge one gets to leave (or have your "tweener" leave) almost immediately after having had sex with your "tweener", regardless or sexual gratification.
(Tweener (adj): People who are sexually intimate with when they are both between relationships - similar to friends with benefits, but with the expectation that while the friendship will continue, the benefits are put aside when one of the people involved gets involved in a romantic relationship.) This is not to be confused with one-night stand remorse or anxiety, since tweeners are not usually just one night stands.
After being out all doing something (day or night) you get home and realize you are horny. You are in between relationships so you call someone you know. The two of you engage in short conversation, have a few adult beverages and get down to business... almost as soon as you finish, you are hit with the "tweener-reflex" and need to get up and get moving or you want your "tweener' to leave so you can sleep. This is not the same as guilt or remorse, its just a feeling you don't want them hanging around.