Hello My 27mo Old Daughter Has A Pratruding Bone Or

Hello my 27mo old daughter has a pratruding bone or vertebrae. Lower back kidney and stomach level. She loves her back rubbed at bedtime and I noticed this couple nights ago. When I touch it she says ouch so its sensitive also. I had many ultrasounds when I was pregnant with her. Had some problems during pregnancy she didnt gain much weight. She was a month premature and weighed 4lbs 1oz. Spent 9days in nic unit. Nothing was wrong with her just tiny. Also had her cesarean. She doesnt have a scheduled dr visit due till june. I will obviously bring her in before then to get looked at. Wondering if you have any advice or comfort. Thanks so much.