It is a combination of a couple of things but basicaly it is related to your growing tummy
This can cause you to sleep in a position that is causing tou some muscle strain
When standing/walking/sitting your center of ballance is off so you hold yourself in a poor position this again puts strain on your muscles and ligaments

To help relieve this you could ty the following
Massage
Hot pack
TENS machine if you have one
Chiro

The reasion I nneded more info is that some liver problems can present as shoulder pains. Don't worry you do not have any liver problems, your answers ruled this out. You have muscle/ligament strain