What Causes Blood Discharge After A Bowel Movement?
Most likely rectal tear but you should get checked.
Detailed Answer:
Given that it sounds like the blood is not in the stool, the most likely cause in your age group is that the bleeding is from a small rectal tear, possibly created by passing hard or large stool. However, given the left lower abdominal symptoms, I think you should go in to be checked by a physician in person.
Causes of bleeding when defecating include tears in the rectal lining and hemorrhoids, but can also be from bleeding higher up in the GI tract. When bleeding happens in the stomach, the stools appear black and tarry, but can be bright red blood if there is a lot of bleeding. So since it is a relatively small amount of blood I think that is less likely. Colitis can cause both the cramping or aching left lower abdominal pain and blood passed with defecation, so I think you should be checked. And colitis can be brought on by stress, which you mentioned. Also, although not common in people your age, it is possible to have colon cancer and this too can cause bleeding.
Your doctor may be able to see if you have a rectal fissure or tear by using a small clear plastic anoscope. This doesn't go in very far, but if your doc sees a tear, then you know what you are dealing with. Your doctor will also be able to see if there are hemorrhoids.
If unable to determine that this is a local (rectal) problem, your doctor may want you to have a colonoscopy. With this procedure, you drink a prep solution that makes you poop until your colon is clean. Then a gastroenterologist uses a scope to see all parts of your colon (via inserting it through the rectum). You will be given anesthesia to sleep during this part, so you won't feel anything unpleasant. There is a CT version of this as well.
I hope this information helps.