My godson has been having very red poo for a while now. It happens every few weeks for a couple of days each time. I work in a lab and have tested it for blood - negative, and I am stumped. A day before the red poo incidents he has been very unhappy and hardly eating. His mum hasn't given him anything with red dye in it.
DS2 does this. Red fruits cause it for him, raspberries, plums, strawberries stuff like that. He loves humzingers and the red ones of those are the culprits almost every time. Other red food does it too even if only apprently smalll amounts (tomatoes, jelly etc). Scared the bejesus out of me the first time I can tell you.