Why would hemorrhoids suddenly flare up for no reason after not being inflamed for decades? i haven't strained or done anything to aggravate them.

Maybe not hemorrhoid: Are you sure you have just hemorrhoids? Pain +/- pruritis (itching) from the rectum that recurs may also indicate anal fissures, proctitis (due to infection, inflammatory bowel disease, radiation therapy, stercoral ulcers, rectal prolapse, trauma, etc.), prctalgia fugax. Depending on your age, risk factors, current medication (aspirin, nsaid's), circumstances, please seek evaluation to be certain.