Milk is not going to be a problem for a lot of people, but for others, it might be the primary cause of ulcerative colitis.
The issue here is relating to lactose. Lactose is a substance found in milk that a lot of the Western world have no problem digesting. However, those that are lactose intolerant (which includes the majority of many ethnic groups), will find that they don’t produce the enzyme lactase in adulthood, meaning that they can’t break down and utilize the lactose. That then causes the lactose to build up in the colon where it can actually curdle and begin causing methane.
This causes pain and discomfort normally, but can of course seriously exacerbate ulcerative colitis.
If you have ulcerative colitis, then it is worth trying to remove milk from your diet to see if that improves the condition at all – seeing as it affects such a large population.