My daughter, 20, has been having low back pain. I found something dime-sized under the skin that moves. What could it be?

Back pain. Your daughter's back pain and the mass under the skin might not be related. It is hard to say what the mass is without an examination. The back pain can be caused by muscle strain, spinal stenosis, ruptured disc, nerve impingement. .. If her pain persists, please see a doctor for evaluation.
Refer to answer. Most probably moving mass is not related to cause of back pain. Avoid push, pull, lift of more than 10lbs. Also avoid bending. If pain persists beyond two weeks refer to your physician. Avoids cereals, juices, sodas, and all corn products.