3 yr old - stomach pains

My 3 yr old complains of her tummy hurting almost every other day.  She was born in the 75 percentile for height and 50 percentile for weight and was in those percentiles until she turned 1yr. since then she has dropped to be just at 5 percentile for height and between 5 and 10 percentile for weight.  I am concerned the two are related.  I was told by my doctor that it may be dairy products causing the tummy problems.  My husband and I are both just under average sizes. me 5'3" and him 5'9".
is there a test to see if it is dairy causing the problem?  Is there something else I should consider could be wrong with her?