S.T.
I have small breasts - but for one of my pregnancies my breast felt like they had rocks in them in my first trimester - into the early part of the second. I had to sleep with a bra - that had never happened before in my life. My milk didn't come in until baby was a few days old and even though I'm small breasted I had more than enough milk for both babies to have almost exclusively breast milk for their first year. (My always starving son did have formula from time to time.)
At 6 weeks you're probably just having a hormonal spike causing the swelling. Pregnancies are different with each baby - don't understand why but they are!