What causes my leg-shaking episodes during sex?

My boyfriend and I have had sex regularly since we started dating 6 months ago. Starting about a month ago, I started experiencing a strange personal phenomenon in which my legs began to shake like rattlesnakes. This usually occurs when we are doing a modification of deep stick (http://www.sexinfo101.com/sp_deepstick.shtml) in which his knees are together and he is leaning further over me (so my legs are at an acute angle to my body) and we are going hard. The shaking usually lasts about 30 seconds to one minute. My boyfriend says I get really wet right after the shaking episodes. Sounds kind of like an orgasm, right? But it doesn't feel any different. One would think if it WAS an orgasm, it would feel at least a bit different. I have never experienced a G-spot orgasm before, but I've experienced the "I've got to pee" feeling leading up to it, and I do not get that before the episodes of leg shaking. What is this?