B., Hi! My name is Dr. Rick Semlow and I am a chiropractor.
If a mother were to bring her child into my office with this scenerio there are several questions that I would ask.
First, does your child "army" crawl? Does he/she crawl only using the arms? Second, does your child stiffen his/her back when you change the diapers? Or does the child let his/her back curve?
Some of this answers may suggest wether or not your child is having back pain. Sitting is most stressful position on the spine. If she is only sitting for a few seconds, she MIGHT be trying to get out of a postion that is causing her pain.
You can find a chiropractor who is board certified in pediatric chiropractic care at www.mcpakids.org
Just one more route to look into.