Large breast after feed?! What on earth is going on?!

This is my fourth baby. Babies have always nursed on every other breast (one breast per feed). This baby is now 4 months old. And just out of nowhere my one boob got humongous - 2-3 cups larger than the other side. I’ve not done anything differently. After feeding baby on the huge side it’s got no more milk left - but stays just as large in size.

I have no signs of discomfort - well maybe just a tinge of some sort of pressing “pain” on a rare occasion but nothing I’m worried about.

I’ve tried researching what this could be online but I find absolutely nothing. Anyone knows what could be going on?! Thanks 😊