When you have GI disturbances that interupt your ability to eat or have BMs normally, your absorption will be affected. For
flare ups, it may not be a big deal to be slightly off for
a few days, but when it's on-going, you can end up malnourished, which can definitely cause fatigue.
It could also be a virus.
Definitely track your intake - especially sugars, gluten, spicy foods, citrusy foods, and acidic foods (coffee included!).... and your BMs.