Having uc and sacroilitis and on medication. Developed ulcer on palet with a dull pain. What could this be?

Immune system. It is obvious that your immune system is inflamed and attacking the mucosal surfaces of the GI tract, including oral mucosa. This is not uncommon with inflammatory bowel disease. The first thing i suggest is try eliminating gluten, dairy and sugar from your diet. In some cases, this will resolve symptoms. You can also apply topical deglycerized licorice or aloe to the ulcers to speed healing.
Stomal ulcer. Oral ulcers are fairly common in ibd. The simplest treatment is kenalog (triamcinolone) in orabase. Ask your doc for a prescription.
Fungal Infx? If you are taking antimetabolites for ulcerative colitis and sacroiliitis, you might have an hla-b27 positive autoimmune condition. The palatal sore can be a component of inflammatory bowel disease (e.g. Crohn's disease) or a fungal infection due to immunosuppression. Your GI and immunology doctors may want an infectious disease consult. Any eye redness could be iritis.
Several causes. There are many causes of oral ulcers. Some occur on their own without other causes ("idiopathic"). However, they can also be due to inflammatory bowel diseases like ulcerative colitis, or certain medications such as Methotrexate or nsaids. Your md can recommend additional testing and treatment based on your specific situation. The ulcers often go away on their own if you can tolerate the pain.