1) what's the meaning of dam becomes milk? I am a baal tshuva, physician. Spent several years in yeshiva and still learn daily. Howdo I reconcile medical halahic claims which medically may not be true but we poskin halaha by them? Notably, saying the teva changed does not cut it, since we still are somech on these shitas lehalaha. 2) what's is the meaning of two makkorot, one of tuma one of tahara in medical terminology?
1) I don't think we have to interpret Chazal's statement literally that blood is converted into milk. What actually happens - and I think this is a very reasonable interpretation of Chazal - is that the alveoli in the breast extract nutrients from the blood stream to produce milk. Please send other "medical Halachic claims" so we can examine them individually and see how to understand them in light of modern science.
2) I assume that you mean "two Mayanos" (opinion of Levi, Nidah 35b). Again, I don't think we're forced to understand that according to Levi, the woman has two physically separate sources of blood. It could mean only that there is some physical difference in the composition of the blood that corresponds with the days of Tuma and the days of Taharah. What this difference is we don't know anymore, just as we no longer understand the subtle differences in the shades of red between different types of blood.
This is not a Psak Halachah