It sounds like the symtoms of a food allergy. Dairy is the most common offender in infants but allergies can be absolutely anything. If eliminating dairy didn't solve the problem within two weeks you are likely dealing with multiple allergies (in my experience that's the rule rather than the exception). The good news is that 90% of food allergies are to 7 things: dairy, eggs, soy, wheat, tree nuts, peanuts, and shellfish. So those are a good place to start.
You can eliminate ALL of them for a week (dairy takes the longest to get out of your system, but you've already done that, so a week should give you results), and see if it helps. If it does, you can add them back one at a time (one item a week) and see if the symptoms return (BTW, allergists often recomend that nursing mothers avoid nuts, peanuts and shellfish even if there's no problem, to avoid sensitizing the baby.)
Block feeding is when you feed the baby just on one side for a block of time, say three feedings. Then just feed on the other side for three feedings. That way the baby gets more hindmilk and not so much foremilk. Too much foremilk can cause symptoms like gassiness and green stool, but I haven't heard of it causing blood and blood in the stool is always a big concern, IMO!
If you search for infant food allergies on the net you will find a ton of info (lots of it conflicting!). The bottom line is that if it's a food allergy (or allergies) it's to a food you're eating and finding it might be tough, but it CAN be done. Good luck!