I can understand that you must be feeling very distressed. Cause of vaginal itching can be due to infection. As you are suffering from ulcerative Colitis, your doctors might have given you steroids to control the problem. That itself indirectly can alter vaginal PH and cause recurring infection.
Next Steps
Along with vaginal medication for local application, go for Vaginal probiotics. Get yourself tested for
diabetes.
Health Tips
Whenever you feel like you are starting with itching, wear loose clothes, cotton under garments.