Thank you for the query.
The most common reason of the blood in stool
are hemorrhoids. Such blood is usually bright red and only on the surface of the stool. But it is very important to make sure that this is only hemorrhoids causing such blood, That is why you should have colonoscopy
performed especially that you do suffer from constipations as well. This test is very important to rule out any other reasons of blood in stool like colon polyps or colon cancer
Hope this will help.