18 year old male, havent had a poo in three days and when i finally did there was blood on my stool i was fine for two days till blood appeared again?

Bloody stool: Constipation & straining can cause anal fissures & hemorrhoids leading to bloody stools, but other conditions such as inflammatory bowel disease, krohns disease, polyps cause rectal bleeding. You need to consult a gastroentrologist. At the mean time try to control the constipation with high fiber diet, plenty of fuids, use miralax (polyethylene glycol) or miniral oil. If the bleeding is heavy go to ER. Good luck