Gastrointestinal stromal tumors (GISTs) grow from the muscle wall of the stomach or intestines. They start small and quiet, so early clues are easy to miss. Internal bleeding is the first red flag. Stools turn black and tarry, or you feel dizzy when standing because blood is slowly leaking inside. Vague belly pain shows up next. It feels like a dull, heavy ache or cramp that comes and goes and doesn’t link to meals. Fullness shows up early. You feel stuffed after only a few bites, or the stomach feels heavy even when you haven’t eaten. Weight loss is common.... Learn more