ajay
^Once punched Jeff Kennett. Don't pick an e-fight
I know shortly after this bit you mentioned not doing bent over fly's, but I found that a particular method of bent over flys (assuming I'm thinking the right thing when I mention that) helped me greatly from an overall 'good back' point of view.
Got smashed by a car on my bike prob 10yrs ago, turned into me as we rounded a corner at same time and I went over bars and landed shoulder first on the gutter at speed. Didn't feel too bad at the time but it got really bad over the next few weeks to the point I couldn't use the whole arm and the pain between my shoulder blades was intense. A very clever physio who specializes in shoulders (and who played water polo for Australia too, apparently) stuck some muscle activity sensor thing on my back between spine and shoulder blade on each side, and I did bent over fly's. The side I hit had zero activity compared to the other, indicating my body was compensating by using the incorrect muscles. It was quite fascinating, learning the body can do this.
Anyway ... digressing a bit. For several years following this, I would get almost the exact same back pain scenario you talked about (pop, and then dull pain), and while it's impossible to know if they're related in any way (and likely aren't) this exercise always helps me. It's a little bit pilates-esque.
After physio dude showed me the screen with numbers on my left side, and donuts on the right, he taught me to try doing the fly action without using any muscles other than the one between spine and shoulder blade. It came naturally on my left, but I literally had to shut my eyes and concentrate to get digits showing on the right. This is with just a 3kg dumbbell. Obviously, you do use _some_ muscles other than the one I'm taking about while doing flys, but concentrating on using this group of muscles both helped my injury, and subsequent 'twinges' and also had left me feeling more confident in back strength exercises (and other back & shoulder actions, like throwing kids up into the air) ever since. To the point I work the exercise into pretty much any routine ... always with a light weight, always slowly and concentrating on posture, the action and the muscles used. My back thanks me.
I do pretty much the same exercise but lying down on my stomach. Chest slightly raised so my nipples are just off the ground, holding a couple of 2.5kg plates with my arms out beside me. Using the the shoulders to lift the weight, 20 reps, 3 sets a couple of times a week. That, along with rolling on one of those physio balls pressed under my scapula seems to keep my shoulders/upper back in check. They used to cop a hiding from all the overhead work. Purely anecdotal, of course.