Why have i started twitching?

i have been twitching alot latley- i dont know why usualy its when i am reading or texting right before bed- my shoulder will jump of my arms will jump forward or my hand will twitch to the right. and yesterday i saw something realy shoking at lunch (2 girls making out) and ever since then when ever my mind remembers it i will star twitching... i mean i usualy shake my head kinda like a dog (its symbolic- i do it when i want to "shake" a thought out of my head) but come on- this is crazy.