A. It could be something as simple as you "slept on it funny" or may be more serious like a tear. Of course I recommend consulting with your physician to seek proper medical attention. Without an X-ray or MRI it is not possible to know the extend of damage, if any in the shoulder. Many shoulder injuries consist of impingement. a tear or "frozen shoulder". A physician and physical therapist can diagnose and recommend a proper course of treatment. In most cases a physical therapist will give you some exercises and stretches to rehab the shoulder. In more serious cases, surgery may be needed followed by physical therapy.
The basic course of action is R.I.C.E.
Rest -Allow the shoulder time to heal and any inflammation to subside.
Ice - For the first 24 hours, apply ice to the injured area. Never put ice directly on the body as you may burn the skin. Wrap an ice pack or my favourite, frozen veggies in a towel and apply to the sore area. This will minimize inflammation.
Compression - Gently wrapping the area for support may stabilize and minimize further injury.
Elevation - In the event of an injury to an extremity such as arm, leg, hand foot etc, elevation will help reduce swelling.
With that being said, when strength training, avoid any overhead pressing movement. When I have a client come to me with a pre-existing shoulder injury, I always focus on rehab type exercises such as shoulder abduction and adduction work as well as increasing the range of motion in the joint without placing excess stress on the tendons, ligaments and joint. I would avoid lifting any weight above shoulder level and focus on working within the client's range of motion.
I strongly encourage no overhead press exercises. These would include;
Overhead shoulder press machines ie. Nautilus, Life Fitness etc.
Barbell overhead or military shoulder press
Dumbbell overhead press
Cable overhead press
Upright row
When performing exercises for the upper body, I recommend performing exercises where the shoulder is supported. For instance, if performing a chest press, I would use a machine press and would use the grip which allow the elbows to track along side the ribs. If performing triceps exercises I would do a tricep press down which again allows the elbows to remain against the ribs minimizing shoulder involvement. This will isolate the specific muscles being worked and support the shoulder girdle. Be sure to perform each rep of every exercise slow and deliberate to prevent further injury. The shoulder is a multiplane joint and vulnerable to injury. If you have a sore shoulder, now is not the time to try for a personal best on the bench press! Think rehab rather than power.
A simple rule of thumb: Listen to your body... If it hurts, don't do it!
*Always consult your physician when an injury is present. This information is intended for general guidance and not intended to diagnose or replace physical therapy.