Hard to tell without seeing the pack on your body but my first guess would be that you're carrying too much of the load on your shoulders and maybe not sharing enough with your hips. Was your hipbelt tight and riding above your hips? Did you shrug your shoulders before tightening it?

Another thought is pack sizing. Yours only comes in one size, for 14"-19" torsos, per the specs. At 5'-6" you should fall in that range but nevertheless it could possibly be too small for you, which means more stress on the shoulders. Do a quick read on pack sizing and measure yourself.

Finally, as far as I can tell your pack is designed without load lifters, small straps that lift the pack's shoulder straps off your shoulders to relieve pressure. If this continues to be a problem you may want to check into a pack with that feature.