Last week when I poop I found a blood with it and whenever I eat something it gives me stomach ache on the left side what is it?

Several things. Possibilities include constipation, diverticulitis, diverticulosis, or possibly a colorectal cancer. An evaluation with your physician is warranted, and possibly a colonoscopy.
Left sided pain. Vine at your age, this is most likely constipation. However, if the pain does not resolve with a bowel regimen and return to daily bowel movements, then get evaluated by your doc. You may need some imaging studies, like x-rays, or colonoscopy.