I was diagnosed last year at 57. I have spells of tiredness mostly when I'm flaring. Even then I try to do a minimal amount of exercise, maybe 25 min on my stationary bike at a really easy level or walking my dog. I also go to yoga class twice a week, sometimes dragging myself there. I feel so much better after the class I can't figure out why I didn't want to go!
Hopefully this will pass for you soon, but in the meantime I'd suggest you try to do just a little exercise, then check yourself afterwards... do you feel better, the same or worse? If worse, maybe a quick check with the doctor would be a good idea.
Potato... Female, 58, dx 11/07 mild left sided ulcerative colitis (50 cm)
asacol 3 3x a day, cortenema (generic)
am & pm rowasa every other day culturelle, benefiber wafers generic acidophilus, metamucil wafers, activia, primadophilus reuteri
children's vitamin, Vitamin E 400 mg, omeprazole 2xday
oxazepam when needed, lomotil or ImodiumAD (not in the same day) when needed
Yoga, meditation, The Work by Byron Katie, exercise at least 5 days a week