Could be your back is compensating for your boobs. I noticed that mine got bigger and did it earlier with each pregnancy.
It sounds like a sore back. Try having someone massage your back. I get that a lot pregnant or not pregnant. It will hurt a few days then stop. It could also be due to your posture and how you are holding baby. Heating pad will help if not on too high. But they massage is an awesome treatment. ( massage therapist )