It seems like you may have irritable bowel syndrome
with alternating diarrhea and constipation
. It can occur after a viral gastroenteritis
("post viral IBS"). More bran and fiber supplements (e.g. metamucil) may help. However since you are already 50 with a change in bowel habits, you definitely need a colonoscopy
to make sure nothing else is going on, and exclude dangerous problems. In the meantime you can also try over the counter probiotics, which occasionally help. For constipation I like Miralax (also OTC), as needed.