vibrating penis like a cell phone vibrating

Three weeks ago i thought my cell phone was vibrating in my pocket. what i learned was that i didnt have my phone with me. I was a shivering feeling in my penis much like a cell phone vibrating. The sensation is not painfull and I don"t have any other symptoms, But it's extremely enoying. What do you think is causing this to happen. Oh yea and sex either brings it on or sometimes releives the vibrating