Hi and welcome to the forum! I am glad that I can help you.
You would like to know the cause of bleeding from the alimentary canal.
The reasons for bleeding in the lower part of the bowels are: infections, strangulations of polyps, fistula and constipation that leads to the formation of hemorrhoids and fissures. Increased body temperature, cramping and diarrhea are symptoms of the majority of bowel infections.
According to the data that you describe, it is likely that you may be experiencing any of the conditions that are caused by constipation. You may consider changing your diet by consuming more liquid-like meals such as soups to avoid constipation. You might also consider increasing the intake of water to avoid tiredness and dizziness while visiting the bathroom. If you experience severe symptoms and discomfort you should see a gastroenterolgist for further advice about diagnosis of your current condition.
Please keep in mind that I provide medical information only. I am not able to diagnose medical conditions online. Please contact your doctor or other qualified healthcare provider such as a gastroenterologist for further advice and information about diagnosis and treatment options.