Arnica will relieve pain, but in my experience it does not correct anything. I am having good results with Blue Emu, which contains glucosamine, chondroitin, msm, collagen, allantoin, etc. It absorbs quickly and deeply. I recommend seeing a doctor who will use ultrasound to examine your shoulders and read exactly what is happening in there. My doctor is called an ultrasound radiologist, doing investigative ultrasound therapy. He can see pockets of inflammation, scar tissue, whatever, and if you need a cortisone injection or lidocaine, he can see exactly where to put it with the ultrasound image.
I have found that setting my bench in front of an open door, and reaching behind me to hold the doorknobs is the perfect stretch for my shoulders. You have to hold a stretching position for more than 30 seconds for the fibers to loosen.
The Blue Emu has helped prevent my tennis elbow from recurring after flare-ups. Exercise, unfortunately, can exacerbate all these problems. I have had very little help from most physical therapists. Some are okay. A good massage can do wonders. Also a sensitive and skilled trainer. “Body Workers” can seem to get too much into their personal theories and interpretations of what you are experiencing, and color it with emotions, so I don’t automatically recommend that approach. There are movement coaches like Feldenkrais or Alexander that may help.
Changing your bench, lowering it for a while may provide some relief. Make sure your shoulders aren’t raised or hunched at all. Even when they are down, it’s amazing how tense they can be. A muscle relaxer like baclofen can also be helpful, along with any kind of anti-inflammatory medicine you can tolerate. It may not be serious if you are young, but if you don’t get to the bottom of this, it can become chronic and a real problem in the future. I don’t think diet is the issue. You can also be terribly stiff from a build-up of lactic acid in your muscles from heavy use without massage. Massage will clear that up.