Hi Mary, Welcome to HealthcareMagic Forum.
You should be seeing a Orthopedist for this. May be it is Costochondritis
. You may need analgesics and other Medications. Kindly get yourself examined by a Orthopedist to rule out the exact cause.
and get adequate rest. Apply heat pads where you get more pain, several times a day and avoid sleeping on the affected side.
Wish you Good Health, Take Care.