My midwife told me LO regulates milk by stimulation. When latched on baby will create what he needs for next feed. When self expressing breasts can become over stimulated. I was advised hot bath and massage engorged breasts. Express until empty and start again. If expressing only express everytime baby feeds alternating each breast so they both don't become engorged.
It's really not fun had similar issues. It astray you have so much milk for your little one but not so much fun for you. Have a look at the Kellymom website for over supply and how to deal with it. Your supply will calm down at some point but if not you can use se I the tactics from there. Mine calmed down after about 12 weeks I then did se block feeding to reduce it further.