Hello! Thank you for the query. This lump can be a sentinel pile which very often associates anal fissure. However it can be also rectal polyp or even colon cancer (rather unlikely at your age). Anal fissure is some kind of wound in the rectal mucosa caused by constipations and hard stool. It gives pain usually when passing stool. Bright blood in stool is possible. Sometimes hemorrhoids (when gets clotted) can give rectal pain. Prostate inflammation can be also a reason of rectal pains. Rectal polyp can give completely no symptoms, sometimes it can bleed (depending of the size of it). And every rectal polyp should be removed as it can potentially become a cancer. I suggest you to visit your doctor and have sigmoidoscopy performed to have clear situation. If fissure will be found it can be treated with diet and medicines. You should avoid constipations (increase fiber rich products and mineral water intake, avoid fatty foods), do not push hard on stool, have sitz baths with Quercus Cortex, use suppositiories for hemorrhoids and ask your doctor about Nitroglycerine gel. Such treated fissure should heal in a week or so. If not, you may need surgery for permanent cure. Hope this will help. Regards.