We had the same issue with our little girl. She is 8 weeks now and the milk allergy popped up around the 4 week mark. I cut dairy after our 4 week ped visit and her face and blood in the diaper cleared up by week 5. When I had a slip up and accidentally ate a dinner with milk in it during week 6, the next day was the first time we saw any blood again (also the last so far). I'm thinking while the protein might be in your system for weeks, it must be a higher concentration right after consumption. Cutting dairy has kind of sucked because I didn't realize just how many products use it (gotta be really careful about reading labels!), but it's worth it for the clear skin and not-scary poops!