For the past few years, I have harsh stomach pains. It would last from about 7:30 AM (after breakfast) to 12:00 PM. However, I hardly have this problem on weekends.
The only difference I have ever noticed is that I hardly ever drink milk during weekends. During weekdays, I usually drink one cup of milk in the morning before going to school.
On the other hand, my guess might not be right because I never have this problem when I drink milk at other times (not during morning of weekdays).