Why do my legs shake like crazy?

Okay, this is probably a little to much info but I'm curious about it so I have to ask. When I'm having sex my legs start going crazy, like they shake violently. It's really embarrassing, I was dubbed thee Thumper. Yeah, I know, psssh, whatever, I make it work, ha ha. Anyways, is that a normal reaction? Or maybe I'm just a little weirdo? llc Anyways, any of your thoughts about this would be appreciated :) And y'all please don't be childish! If you're a child answering a sex question, uhh, seek Jesus in your life, just kidding :) Ha ha but seriously, be mature! I just curious if it's a problem that happens to a lot of people. So, on with the answers my lovely yahoo answerers! :)