I'm no Dr. so take what I say with a grain of salt here, but if the pain subsides and then returns under a heavy workload this sounds like definitely a joint issue, not a muscle issue. I had an issue with my A/C joint in my shoulder some years back, and it came from doing extremely heavy military presses with dumbbells. It was what is termed "impingement injury" which means basically the cushion in my joint was wearing thin due to heavy weights causing the joint to rub against itself. It really bothered me sleeping on it more than anything. I cut out military presses for about 4-6 months, and took glucosamine and fish oil, I think I trained with alot of machines during this period as well, keeping my elbows in and doing 3/4 reps. The common cure for this is a shoulder scope, where they go in and clean the joint out giving it room to move freely once again.Mine felt like something was clicking or pinching inside my shoulder and limiting my range of motion. I would recommend going to an orthopedic Dr. and having it X-rayed so you know what your dealing with, if its a joint you'll see it on the xray, a muscle will obviously need an MRI. The best advice I can give you is to #1- stay away from whatever exercise gave you this pain, in your case bench pressing. Stick to more incline or decline movements and see if it the pain subsides. Also, machines are great while working through an injury. #2- Any pressing movements go 3/4 motion on, stop the push about 3-4" from lockout, and stop the descent about 2-3" above your chest. This will keep the shoulders out of play and keep it on the chest. Start taking some glucosamine w/ chondroitin and also fish oil daily. It wouldnt hurt you to get a cortisone shot in the joint either, just use the shot to help with the pain, NOT TO TRAIN THROUGH IT. You can cause more damage with cortisone if you simply use it as a crutch to continue training the same way you were. In my case the pain came on all of a sudden, the 130 lb dumbbells was just enough weight to push that joint over the edge and cause pain. It eventually subsided without any surgery, but I had to change things up a bit and train smart. I cant be sure this is your issue, but if the pain is more during a front delt raise then I'm guessing its your A/C joint, the most common joint injury in lifters actually