What Does Swollen Feet Indicate After Intermittently Carrying A Heavy Object?
Rotatory cuff tendinitis
You seem to have injured muscles around the shoulder joint. Generally they are in rest and when they contract for so long time (while you hold your grandson) they tend to irritate at the point where they insert into the bones.
Generally rest relieves the pain. I prefer to give Tab ALEVE /NAPROSYN 2 tablets immediately after breakfast and 2 tabs after dinner each day for 5 days. If you have previous gastric ulcer issues please do not take this.
If it does not get any better or worsens after 5 days I suggest to see a Orthopedist for physiotherapy sessions. They can do ultrasonic therapy to ease the pain along with some exercise sessions.