The tomato skin if not digested properly can look like blood in the stool. Otherwise tomatoes do not change the color of stool. I have heard that if their is blood in the stool, the stool is a blackish color.
That will only happen if the tomatoes are undigested, such as in a case diarrhea. Another possibility is if you ate peppers and tomatoes in such volume that some passed through. Otherwise, it is blood in your stool.
Tomato skins pass through my digestive system as if they were made of plastic. Years ago, they caused no problem. I don't know if I changed or tomatoes have been bred to have tougher skins for shipment to market. Any ideas?