Why was there blood in my poop?

Yesterday I was out all day and when I got home I was really hungry and couldn't find anything good to eat. All I could find was a couple of my mom's weight watchers cuisine things. I took the chicken alfredo one and ate it. Then a while later there was blood in my poop!?! I never experienced this before so I don't know what caused it. Im only 14 and I only weigh 90 something so do you think it was because the food wasn't meant for me it made me poop blood? Please help me!