I noticed that no one has responded to your query yet, so I thought I'd put in my two cents. I'm no doctor, but have dealt with a shoulder injury. It seems to me that the shoulder is the area causing the most problems for weightlifters/bodybuilders, and can become chronic due to poor blood circulation in the area. From what I know, it could be caused from a musculature imbalance between the front and the back of your shoulder girdle (e.g., training chest more than back). It could be tendonitis/bursitis from an impingement in the rotator cuff (i.e., there’s not an adequate amount of room for the uppermost tendon of the rotator cuff to pass beneath the bone on the top of the shoulder, called the acromion ). It could be an instability problem...loose tendons and ligaments (previous shoulder dislocations can cause this). You may also have torn a rotator cuff tendon. With all this said, only a doctor can tell you for sure.
If you see a doctor he/she will probably first recommend that you modify your training routine (rest)...temporarily laying off those things that cause pain...and use ice and take anti-inflammatories as needed until the shoulder is healed. For the long-term, you may want to incorporate some light strengthening and stretching exercises before workouts to prevent future shoulder problems. If the problem persists, you should seek advice from an orthopedist. My biggest fear when I injured my shoulder was the potential need for surgery. However, a good specialist will want to first try every viable non-evasive option available.
There are some good recommendations in other discussion threads on this site. Just play around with the search engine. I have personally had good success with therapy from a good PT. I can now do some light bench pressing and normal shoulder work that I previously could not do without pain. I am now trying a treatment called Active Release Technique. I don't know if I'll be able to bench 450 again, but am optimistic that I can get back to good mass and strength regimen without surgery.
Anyway, I hope everything works out for you. Just use common sense and do whatever it takes (with help from trained medical professionals...if needed) to heal your shoulder. You definitely don't want to do something that makes it worse. A small, temporary drop in your bench max now may pay-off big time in the long run in terms of a strong, healthy shoulder.
Good luck!!