hi, ds kept getting these bruises right in middle of lower / mid back, on his spine - not sure if similar to you or not - someone suggested to me they were caused by him moving backwards in the shopping trolley when we were at the shops!
dd1 got them to, in strange places, not on her back. the more i thought about the more i could find things she was exposed to that caused them. some ppl (like me and my dd1) are extremely prone to bruise, even from the pressure of your fingers on the spine when you lift her up!
the little girl I used to nanny for got them too - we discovered they were from her car seat she was very skinny and the seat used to sort of catch on her back occasionally. We put a pad made from fleece behind her and this stopped them.