Oh dear...maybe it is just ill?
I am not sure as I have not raised CXs. I just wanted you to know that such a bird existed...so you can perhaps determine if that is what kind she/he is.
http://www.mcmurrayhatchery.com/jumbo_cornish_x_rocks.html
If you find out it is that kind of bird, you may need to research it since I believe that those birds have to have food only certain hours of the day???
There is such a thing as a Cornish Cross (meat bird) that gains weight very quickly. You may wish to see if it matches pics of them. You can look on the meat birds section of BYC if needed, or try googling. I will try to find you a link....