Four month old screams?

my four month old son will have one major scream a day. He will scream non stop for over 30 minutes. He isn't hungry, tried, or need his diaper changed. It usually occurs when he is in the cradle position. I have been to the hopsital twice and the doctor three tiems and have been given colic. He is not colic. ON top of the screaming he just wants to be held all day everyday. Which is hard with an 18 month old also by myself. I feel like something is wrong. He has only been doing this for a few weeks but it is pretty bad and gets pretty hard to deal with. ANY one know what i could be?