What Could Cause Back Pain?

My 2 year old daughter (2 yrs 5 months) has been complaining of back pain every day for the last week or so. When I ask where it hurts, she consistently points to her lower back on the left side. She seems to complain more when she wakes up or when she s tired. She doesn t seem to be sick, she s sleeping and eating normally, and lots of energy as usual. However she has had some loose stools and minor diarrhea (diarrhea lasting for just about 24 hours). She has also recently been potty trained, but is still struggling with pooping on the potty, i m worried that she is holding it in and making herself constipated. Could that be the culprit, or could it be something more serious?