Are my legs supposed to do that?

My girlfriend goes really weak in the knees if that's what you mean. If we do it standing up she has to be leaning against something with her knees locked

Yes. It's better than what I experienced, when I first went back to masturbating. My legs and then the rest of my body, were weak for days. I thought I was going to pass out and die from the weakness.

Different people different things.

I think it also depends on the intensity of the orgasm.

some peoples legs go super crazy like they are doing a bicycle kick.

Are you Male or Female, if you Male it can be quite a problem and I for one would suggest that you sit down when you masturbate or even lie down, it will be quite awkward if you should fall over and bump your head against something which could knock you out with your dick still in your hand and cum oozing out of its tip. can you imagine if somebody had to find you in such a position?

Remember, "Safety First, at all times for all activities!"