What causes bloody mucus in stool? when i feel the need to poop only bloody mucus comes out. and when i do poo it's narrow. also feel gassy and nausea

Proctitis? Colitis?: You have some sort of serious inflammation of the rectum (proctitis) or colon i.e. lower intestine (colitis). Most likely inflammatory bowel disease, including conditons called ulcerative colitis or Crohn's disease. Or certain infections, including STDs If you've had anal sex. You need to see a doctor about this ASAP. Good luck!