It could be that you have milk ducts in the arm pit area which is why you’re feeling an ache. This is normal as glandular tissues do extend to the armpit area. Wearing a good and comfortable sleep bra for a bit of support at night night help mama. I also thought this article by Medela is great and I’d like to share it with you: https://www.medela.com/breastfeeding/mums-journey/breast-changes-pregnancy-to-weaning
Fannie Lee