Some BF mothers get a lot of upper back pain or shoulder pain, which comes from carrying the baby, and all the bending over to breastfeed. There is also extra weight on your upper back from the milk and your increased breast size. My upper back and shoulders actually got so bad I had to see a physical therapist. She gave me some great exercises, and I would recommend you see one if your back and shoulder pain is really bad. Don't just keep taking those drugs they gave you at the hospital! : ) That kind of pain is not good or normal!
I also would recommend doing gentle back exercises. You can look these up on the internet. One easy one is to stand against a wall with your shoulders touching the wall, and put your arms straight up against the wall, and then swing then slowly all the way down in front of you, so that they touch the wall again below your hips. Then you go back up, and down, slowly. If you are able to do wall push-ups, those are really good. Or if you can do harder kinds of push-ups, like against a counter or on the floor, even better! You can also lift weights or pull on surgical tubing that has been afixed in a doorjam. Be gentle on yourself because you are extra-flexible after pregnancy. Yoga helps too, but not enough yoga positions really strengthen the upper back specifically, so you might have to do push-ups and weights in addition to a yoga routine. If in doubt, complain of upper back and shoulder pain to your doctor, and request physical therapy.