Why did i shake when i saw him?

What your describing sounds like a good ual attraction and you had a bit of an Eye is what my friends and I call it. Its pretty normal when you are attracted to someone and you both make eye contact and smile. There is nothing wrong with you for having this and there is no reason you have to act ually upon it, but being aware of that feeling is helpful when you do decide to become ual with someone and it makes it more intense.