I thoroughly dislike single strap bags. I had a single strap for three years that i bought for some god awful price and ended up selling it for a quarter that just to get rid of it. I settled for a generic nylon covered rucksack and it took a lot of the oddity of a single strap away. Granted I'm not be able to swing it around front when i need something, but i haven't missed that yet.

Perhaps your pack straps are too tight or the pack is leaning to a side. I had a period where it felt like the double strap was constricting a little bit of my movements, and all i did was release the straps a tad so it sat lower on my back.

Is the bag stabbing into your back at all from odd shaped items? If that's the case, you may want to slide something in the back of the bag so it distributes pressure evenly, or invest in a backpack cushion.