I've noticed my breasts have increased in size quite a bit since I became pregnant, and they feel a little sore too. Why does breast size increase during pregnancy? Is it a normal part of the process?
It's just your body preparing to feed your little one. The milk ducts are growing and blood flow increases, which is exactly what helped me nurse my baby later.
Due to pregnancy hormones like estrogen and progesterone, the breasts swell and feel sore. I had this too and wearing a comfortable, non-wired cotton bra helped a lot.
Hormonal changes are completely behind this. I had thyroid issues so my hormones were already all over the place, but this is a very normal pregnancy sign.
Don't stress too much about the sudden size change, it is totally normal! Your body is just getting ready for the baby, so go buy some comfy maternity bras!