first bowel movement in a week and had blood in stool
Rectal bleeding or bloody stool (hematochezia) may arise for a number of reasons. It could be bleeding within the lower bowel – colon or rectum. Hemorrhoids is commonly seen in constipation and could be the cause of this bleeding, as well as anal fissures. You should report this to your doctor as soon as possible so that further investigations like a colonoscopy and stool analysis can be conducted. The results of these tests will provide a better indication of possible causes. Your constipation will also need to be treated but your doctor will advise you accordingly.
You can find some causes in these articles :