What makes you weak in the knees?

So I JUST figured out that, there is a spot on my back that turns me to play dough. If it's poked, rubbed, scratched, sensually touched It's like ic. So I have to watch out for anyone who is behind me about to hug, pat, whatever. LOL squirm away saying "because I'm ticklish.," When in reality I would moan and my body would spasm. LOL Anyone else have something similar?