What could be the cause of left abdominal tightness and pain?
It appears that you have two issues which may or may not be related. Left lower quadrant abdominal tightness and pain, which occurs episodically through the day, is described as colic. In a 26 year old gentleman, the causes of left lower quadrant abdominal colic can originate from the bowels (intestines) or the urinary tract (ureters). In other cases, the discomfort can be due to nerve related issues (known as referred pain) or abdominal wall musculature, especially if it is related to movement.
Your second complaint of blood stains noted on the toilet paper after bowel movement, suggests bleeding possibly from your anal canal. This is often due to trauma to your anal canal from hard stools, prolonged sitting, or straining when passing motion. However, it is not uncommon for the bleeding to originate from other causes in the rectum or colon.
Both your complaints require a detailed history-taking and physical examination by your doctor to determine the appropriate investigations to confirm the diagnosis. This will then guide the treatment towards the source of the issues, allowing you to achieve a long term resolution of your symptoms, as well as prevent future recurrences of the issues above.