Lying on Side and stomach are actually the worst affected. This is especially annoying as they are my favourite way to sleep.
I made a crappy drawing for you on an online app to show you what I mean:
When lying on side my mid section sinks downward as my hips and upper body are both wider. This causes spine to bend sideways producing strain and pain.
When lying on front the hips are usually half twisted with one leg to the side. (does anyone lie totally flat on their front?) this produces a rotational strain in addition to the bending strain.
Lying on back, especially with rolled up sweater under lumbar area has proven to be the most comfortable. The problem is I cannot sleep like this and end up going to side/front.