Starting about 10 years ago, I quit eating dairy because I noticed that every time I had dairy, my joints would be creaky. After not having dairy except for on the very rare occasion, the creaky joints turned into joint pain the next day following when I ate dairy.
A few weeks ago I accidentally ate a sandwich with cheese on it, and I was completely fine the next day. A week later, I ate some cake with ice cream. Again, I was completely fine the next day. This past weekend, I had a "friendsgiving" event where I ate 2 plates full of food made with milk, cheese, and butter. Again, I was fine the next day.
The only difference I can see from previous times eating dairy is that I have been taking NAC about every other day, and I think that may have fixed my dairy intolerance.