From what I remember that happens sometimes but I don't know if there's a way for you to deal with the obstruction. Can you give your vet a call and see what they say?
My concern would be mastitis (infection); there are other possible explanations, but that's my first thought in a lactating cat. She would need to see a vet, who would be the only one who can diagnose this and it would need to be treated if it is.I haven’t seen any type of milk at all and I’ve tried they all seem to latch onto the other ones. All the other ones look normal. They’re squishy but this back one almost feels like there’s a rock in it