Sounds like a bit of a muscle problem.
The shoulders have very complex muscle systems and as we age we lose muscle tone, yet still try to do things that are really beyond us.
Just part of ageing, I'm afraid!
i get similar and other problems, and my physio told me yesterday that is just because the back muscle are too weak to cope.
So I am doing a daily routine to gradually strengthen them, and hopefully stop the pains.
One exercise is to bend the arms up at the elbows, in front, then move the arms to the sides, forcing the shoulder blades to come almost together.
Do this 10 times, slowly.
also do shoulder shrugs.
I do another one using an elastic type tape, which I loop around something then pull on, once again pushing the blades together.
you may find a remedial massage will give relief, but if like me you just have to get fitter.
Carrying kids is hard on the body, just note what is does to your posture!