Hip pain has been mentioned.  I can make it worse. That has made it possible to make it better. If my pack touches my rump mussel my hips will hurt. Last month near the end of the 2 mile bushwhack from the beaver dam to the Wawiag my hips started to hurt.  I put my hand between my pack and the small of my back and after a while it relived the hip pain.  If I use a hip belt it pulls the pack against my rump and the hip pain soon comes on strong.  A good framed back pack does not do this but portage packs are NOT good framed packs. 
The more expensive portage packs have load lifter straps.  As you tighten them they make the pack load slide down on your rump. I call them discomfort straps.  If your pack is too comfortable just pull them a little tighter. They are meant to keep the pack against your back when you have the load supported from the bottom with a good hip belt which requires a good frame which a portage pack does not have.
If your hips hurt on portages while carrying a pack and not the canoe something is wrong with your pack.
I carry a old Woods Mason pack (one of the cheapest pack on the market) and I sold my Copia bought high dollar pack because the Woods mason pack carries better.