As the body adjusts to no longer having this large object compressing all the internal organs, changing the centre of gravity, and altering spinal and rib mechanics, the curves in the mid- and low back start to return to pre-pregnancy proportions.
However, this is complicated by a few factors.
Change in bust size and weight increase the forwards pull from the breasts, leading to increased strain on the thorax, particularly the upper part.
This generally leads to a flattening of the thoracic kyphosis (outwards curve), increasing stress on the joints in the spine, forcing the muscles that hold you upright to work harder, and generally reducing mobility through the area.
The above-mentioned reduction in movement in the thorax quite naturally makes the areas around it work harder.
This can be particularly noticeable in the shoulder, because it relies on so many things working well to do its job properly (for more information on this bemusingly fragile joint complex, see the relevant series of posts).
You will no doubt have received oodles of advice about breastfeeding, which position is best, how to get a good latch, etc, etc.
The only two things I would like to touch on here are the importance of bringing the baby to the breast, rather than leaning the breast into the baby (this is regardless of position, generally), and the fact that it will probably hurt quite a lot, at least initially.
I know this does not sound like a problem (and I am certainly not advocating you avoid it!), indeed, one of my wife's biggest joys of breastfeeding was the sense of connection with this little being you are nourishing; but holding your head in such a flexed (and normally sidebent/rotated) position puts a huge mechanical strain on the shoulder, thorax and neck
This has the advantage of leaving your dominant hand free. It also means you are doing a lot of unaccustomed work with what is probably your weaker side. This may result in elbow, forearm, wrist or hand issues.
Carpal Tunnel Syndrome is when a nerve gets compressed in the wrist. It is very common during pregnancy due to all the extra fluid sloshing around, which takes up valuable real estate, particularly in small bony tunnels (like the eponymous carpal tunnel). Although this should improve after giving birth, these symptoms may continue for a while, particularly if you were getting them on the side on which you now hold your child.
This is very closely related to the factors discussed above involving upper limb, shoulder and thorax. In particular, the reduced range of movement in the thorax and ribs means that all head movements get 'focused' into, well, the neck itself, rather than the load being shared down through the rest of the thorax.
Add to this the Baby Adoring Gaze Syndrome, and we have the potential for a very stiff and sore neck.