My son had blood in his stool as a newborn. We eventually narrowed it down to casein intolerance (dairy protein). (He is also soy-allergic. He's four years old now, and outgrew the dairy problem, but soy is definitely still a no-go.) It can take several days to a few weeks for the dairy to clear your system. However, if you've eliminated dairy and the diapers are getting MORE bloody, I think I'd be alarmed. It may be that something more is going on here. It could be that you are still getting some of the offending foods somehow (look for hidden dairy and soy in packaged foods--- it can hide out under many names). It could be that it's a different food that's causing the issue-- eggs, for instance, or something else. It could also be that this isn't a food intolerance at all, and further testing is needed to determine what's going on. I think you're right to be looking for medical attention-- I would be doing the same.
Overactive letdown would not be my first thought here. Blood in the stool signals an injury to the intestine. Overactive letdown often leads to too much lactose, but the usual symptoms of that are green, frothy stools, not blood. Sometimes the cause of blood is fairly innocuous, but sometimes it's not, and it's worth taking very seriously.
I sure hope you find out what's going on here soon, and that it turns out it's no big deal. I was so worried when my DS was having bloody diapers, so I can imagine that you must be so anxious.
