Hello, In The Past Year, My Four Year Olds Mid-lower

Hello, in the past year, my four year olds mid-lower spine has had a bone seem to stick out more that the rest of his spine. When he stands, you can see a small bump, but when he touches his toes, the bump is very noticeable. He says it doesn't hurt at all. It also appears to become darker on the skin if he sits in a car seat for a longer period of time. His pediatrician ordered X-rays, which were normal, and I haven't been followed up with since. I am very worried. Is there any other possibilities, or is this normal? thank you!