Like the others said, hard to tell wihout seeing you wearing it, but it could be the weight distribution between your hips and shoulders.

I sometimes can adjust my straps so the pack weight is almost entirely on my hips, but usually wind up about 50/50 or so.

Try easing off on the shoulder weight to start and see what happens.

As TR says below, check the pack size. The TNF website has a chart-look at that and compare the recommended size to you and your pack. It may be too short and not riding on your hips right.

Also, if your shoulder blades are hurting, you might have the pack cranked down too tight or the stays are not bent to your shape. I try to wear mine just tight enough so it doesn't rock around.

Try taking the aluminum stays out and laying them against your back in the position they would be in the pack-the contour should match your shape. If not, tweak them a bit-which may take some doing-so that they seem to parallel your back while standing up straight.

btw-the TNF chart doesn't show the pack has stays, but the pack description does, so I presume the specs are correct, not the chart.