Look at your diet and see what red things you are eating. If there is nothing, talk to your doctor. They may need to run some tests to make sure you aren't having colon issues.
I don't eat watermelon and it oftens oftenish and I don't eat a lot of tomato things, that I really know of.
It's probably something you are eating. If you eat tomatoes or watermellon, that I'd a common occurrence