Just a thought - can you rent pack goats or llamas where you're going?

I think I would have to see what the pack/straps are doing to make a good guess. I'd suppose that something in the fit of the pack is off. I bought one of the Granite Gear Nimbus line of packs for this reason - the frame will allow you to increase or decrease the width of the shoulder strap attachment, and they will replace the hip belt and shoulder straps with whatever level of padding or size you need, women's or men's versions, for free. I have found that the small pack frame and medium hip belt/straps works fine for me.

If your pack is not one with removable straps/belt, finding somewhere you can have yourself measured and try on different harnesses with the proper pack sounds like a good thing to try. Recently someone I know went back to be re-fitted - turns out the problems he was having was due to someone not measuring him properly and selling him a wrong sized pack.