Vitamin D is vital for immune health. It has been suggested that the "seasonal flu" that appears every winter is due to generally low vitamin D levels in people every winter. Crohn's interferes with vitamin absorption generally and vitamin D which is absorbed in the small intestine is particularly hard hit. To raise your vitamin D stores you need to take 2000 IU daily, but you may want to take more for a short time to boost your stores. I would recommend a product by Biotics Research which is called Bio D forte (see: Biotics Research
It seems odd that you are on folic acid, but not B12. Generally B12 is more likely to be a problem in Crohn's. Is this your doctor's suggestion or did you add folic acid yourself? Has the doctor done blood tests for vitamin/mineral levels? I was going to assume that your doctor had tested for iron, but I began to wonder if that were so? In any event, many people find ferrous sulphate to be very hard on their gut. It is much better to get a chelated form like ferrous citrate because it is easier to absorb and not so hard on your gut.
Finally, you don't mention taking any probiotics. They are one of the more useful things you can do for your gut, if you get the right ones. The most effective ones by the results of actual testing on people with intestinal problems are: Culturelle, Align and VSL#3. The first two are in many drug stores - don't know about