Hi S., I just joined mamasource, and your question got my attention, I was in the same situation a year and a half ago. Amazingly my daughter started some rectal bleeding also at 4 months, we went crazy trying to find out what it was, until we finally (with a gastrointerologist) found out that she was allergic to milk.
Anyways, you can start seeing changes about 2 to 3 weeks after your diet changes, becuase it takes your body 1 week to adjust, 4 days to process into the breastmilk (I also breastfeed) and one more week for your little one to process all this changes into his/her body. Believe me I went to every doctor you can ever imagine, even a breastfeeding consultant, one of the best helps I ever got. But be aware not only avoid regular milk, but also cheeses, butter, chips, or snacks that have milk in it. breads, pastries, etc. there are a lot of foods that have hidden milk in it, and while you are eating them, they will still go into the breastmilk, don't stop breastfeeding, since you are incresing his/her inmune system that way. Just be patient and things will get better. I used to have some websites that helped me out a lot to know what products and baby food had milk in it. I'll look for them asap and send them to you asap. I am glad I can help somebody that is in the same situation I was.
Take care, and congratulations for breastfeeding your baby, not too many people care about the benefits that it brings to the new baby.
C.