It could be the fit of the pack, both in it's size relative to your torso and how you're wearing it. There's a video and article here: http://sectionhiker.com/how-to-fit-a-backpack/

When you put on a pack, the proper order for tightening everything is hip belt, then shoulders, then load lifters, then sternum strap. The hip belt should be just above your hip bones, and support most of the weight of the pack. When viewed from the side, there shouldn't be any gap between the shoulder straps and your shoulders. If you adjust the hip belt correctly, but are unable to adjust the shoulder straps for a proper fit (or vice versa), then there's a good chance your pack is too short or too long for you. The height of the shoulder harness may be adjustable.