What causes small black balls the size of a pea to show up in your poop? what should i do?

These could be undigested fragments of certain food you ate, but could also be from bleeding that occurred in the upper gastrointestinal tract, specifically esophagus, stomach or duodenum. If they persist your physician can do a simple test in the office to determine whether occult blood is present which would require an evaluation to include upper endoscopic examination by a gastroenterologist.