Could be constitutional related but colonoscopy is required

Rectal bleeding cannot occur after radiation for throat cancer... Kindly consult a surgeon in your vicinity to rule out other causes of rectal bleeding... Take care

Radiotherapy to neck has no correlation with rectal bleeding. Will need to assess for cause of rectal bleed

Not directly related to radiation.. Have u detected any diet changes like decreased intake..could be due to poor diet.. If the quantity of bleed was very less with constipation and hard stool u can start using syp duphalac 30 ml three times  a day for 2 days.. Increase diet and fiber intake If severe bleed with regular stool meet a gastroenterologist or treating doctor earliest