Congratulations on your new baby!
Did a great job in describing the painful sensation that you are feeling in your jaw and neck.
One nerve supplies the areas that you are describing. It is the mandibular division of the trigeminal nerve, cranial nerve V. Small branches of this nerve work to provide the pain sensations in the area and also move the muscles in the same area as well as supplied the skin in the same area.
It is not uncommon to have postural changes affect this nerve. You did mention whether it is getting better or getting worse. In either case it needs to be checked out.
I recommend a comprehensive examination by a TMJ/neuromuscular pain management specialist. In my experience a number of conservative measures, when done correctly, can alleviate your problems. Dr. Paul