chick just died..crop felt play doh like...?any suggestions?

One of my jersey white giants (going on 3wks now) was sitting on the floor of the brooder, the other chicks were up and acting normal. I picked her up and she didn't "fight" at all to move/get away. Within. 5 mins she died in my hands.while she was sitting she pooped, it was slimey, yellow, and stringy. Her crop felt like play doh. They were all acting normal yesterday. anyone know what this could be? I have two heat lamps in the two story brooder. They use a ramp, its not very tall or anything. They have plenty of room to get away from the lights. And clean water, clean food. I am very scared for the other chicks now.