C.
Could be a lactose intolerance or about a dozen other things. My son has a lactose intolerance and the symptoms match, but you may still want to consult with a GI doc. You should, as another mom suggested, keep a food diary. Write down what she eats and when (if you see a GI doc they will be happy you did this). You may find an obvious culprit. Because lactose intolerance is common, you may want to try removing it and see what happens for a few weeks and then try adding it back. Or, you could do a true "elimination diet" where you remove all items known to cause allergic reactions (wheat, dairy, etc). After a few weeks of cleaning out her system, you start adding foods back slowly.
Good luck.