By seeing all the symptoms that you have mentioned above it looks like you are suffering from fissure in ano secondary to anal irritation which is evident by the itching that you have. I theblood in stool I guess must have appeared after the itching,if so it is suggestive of a fissure in ano....Considering your age it is common in people who consume a high quantity of food the is deficient in fiber content also the anal irritation could indicate a worm infestation which too is quite common in people who eat outside Food which most often isn't cooked fully...

Blood in stools along with pain and itching could be because of many reasons, but the most common cause of all 3 together is anal fissure. Please get yourself examined by a proctologist.