My usual cycle length is between 31-38 days and would last 5-7 days. this month my cycle came at cycle day 41 and last 3 days. what would cause change?

Normal variation: This is only 3 days longer than you would consider normal so it can still be some of the normal variation in cycle length. It can also be due to stress, changes in your schedule, sleep changes, exercise changes, diet changes,weight changes, medications, or illnesses. Your cycle is like the canary in a coal mine and is very sensitive to pretty much every thing. Hope this helps.