yes, I think this is possible, but no idea bout the time period. Over a period of wees/months my dairy-sensitive toddler reacted to trace amounts of dairy we were letting him have. He was having bread on a daily basis in which the final ingredient was milk. No noticeable reaction after just a few days, but over a period of weeks/months it started to affect him. Then it took weeks for us to sort out the problem once we realized he was reacting and then the reaction cleared within about a week of eliminating the offending bread.
