We're prone to B-12 (folate) deficiencies and iron deficiencies as UC patients. Folate-deficiencies do cause fatigue, just as much as iron deficiencies do. We can be genetically-predisposed to have low folate absorption from our food. We can be real selective in what we eat and might have low folate levels as a consequence thereof. Not sure what you're eating, but meat, fish, dairy, broccoli, brussels sprouts, asparagus, peas, chickpeas and brown rice are all good sources of folate. Always prefer natural sources over supplements whenever possible.
/www.nhs.uk/conditions/vitamin-b12-or-folate-deficiency-anaemia/causes/