My 3 year old isnt eating like she's suppose to, but isnt in any pain . what could this be?

Could be normal: Many three year old children seem to live on air. This could be a normal developmental event.

Growth chart: The growth chart is helpful in evaluating her progress in gaining weight and height. If she is following a growth curve, then it is likely normal. Many toddlers will eat small amounts of food frequently rather than one larger meal. Offer healthy foods and glasses of milk during the day. I would suggest reviewing her growth chart at her next office visit.