I too only experience any lactose problems when I am having a bad flare. I guess this makes sense as lactase (enzyme that breaks down lactose) is present in the intestines, and I guess while a flare is occurring there is probably not enough lactase being produced? I am just guessing based on my own symptoms and experiences, but talking to your Dr. is the best thing to do.
I can still usually eat cheese and yogurt (frozen yogurt is one of my go-to-foods during a flare!) as the lactose content is low, but milk and high milk fat foods just don't work. I just trade my regular skim milk in for lactaid milk during the flare.
My GI said it is totally possible to become lactose intolerant with Crohn's though from the constant irritation, so getting tested or doing an elimination diet is a good way to go! I now see a dietitian and keep a food journal, and it really helps during flare ups