I saw melted cheddar cheese that I ate in my stool. I also have stabbing pains in abdomen and lumps in rectum? What are the causes?

Need more info. It is unlikely that melted cheddar cheese made it through your intestine unchanged. However, cheese does lead to constipation in a large number of people which can cause abdominal pain. Constipation can also cause thrombosed external hemorrhoid presenting as lumps at the anus.