Heather Corinna replies:

Well, it's entirely possible that that trembling IS orgasm, or at least the start of orgasm. If it's the start, then the trick is to just keep on doing what you're doing at the moment and see it through.

In general, it is often sage to say that you know when you have an orgasm. But one thing we've noticed over the years here is that so many younger people base their idea on what orgasm looks like or seems like on pornography or popular media, that it's common for a person to be reaching orgasm, but not recognize it as such because their expectations of what orgasm is like aren't realistic.

Check out this piece on orgasm and sexual response, and see if that doesn't help you out some. It is typical, with the advent or orgasm or plateau -- which leads to orgasm -- to feel trembly much of the time, so my feeling is that you are either there already or thisclose to there.