It sounds like you need to take a few days off rather than an equipment issue. The AG100 may be stiffer than the PSL, but it's still a relatively flexible racket. If you transition to a racket that you need to swing out on even more while exerting more energy, it'll probably only make your shoulder pain worse. For me, I use multi strings and would string at or around 65 lbs consistently and usually don't have any physical problems playing with the AG100. If you're stringing at 55 lbs, you shouldn't have to swing too hard to make the ball go.
Another alternative you could consider is maybe acquire a couple of used AG300 16x18, which is what I'm using now. It's the next most similar racket to AG100 but the stringbed is more forgiving due to the larger 98 sq in headsize. Otherwise, the weight, balance, and swingweight is very identical to AG100.