Answer:
If she is otherwise growing well and has no abdominal pain, flatulence, or diarrhea when on a diet excluding only lactose then it's quite unlikely that something else is going on.
It may or may not help but will certainly not harm her to try adding a probiotic to her diet for a month or two and then trying lactose-containing foods again. Start with a small amount of lactose.
Check with your pediatrician if she continues to have these episodes of gastrointestinal distress.