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Colon swelling, weight loss and rectal bleeding
all indicate towards a serious illness affecting the gut.
Prominent possibilities are abdominal tuberculosis
, enteric fever
, amoebic dysentery, Crohn's disease, ulcerative colitis
to name some of the putative diagnoses.
To get a more clearer diagnoses you should undergo a colonoscopy
and if needed guided biopsy of the gut. Also get a blood work up CBC, RFT, CRP, ESR, LFT and USG Abdomen.
If you are masturbating towards anus then you might be moving the affected area to precipitate bleed.
If you have any further query please let me know I would try to answer it for you.
Wish you a great health!