If you are worried, you can do 2 things. 1 - make an appointment with the Ped. and have him/her explain why they think it is that simple. If they have had experience with it, and this type of occurance is valid, they can show you. If not, then GET A SECOND OPINION. Right away. Many times, a parents worry is just that - because it's a new situation and it involves your child, and it's natural. However, too many times, a over worked doctor, an inexperienced doctor, or one who is just too wrapped up in themselves will not do what they should - really pay attention to the patient, and when it's a young patient, to the parent(s) as well. It probably is from the stomach ailment. Cells passing through from the intestines and bowel after so much trauma. Also, if she was lactose intolerant until just a short while ago, remove milk products from her diet for a couple weeks to let her system relax. Then SLOWLY reintroduce them. Like a small (4 oz) glass of milk every other day until you are certain she is going to tolerate it well. Remember, intolerance can & often does resurface. And after an illness, her system is not up to par. Did you give her pedialite or other drinks to help balance the electrolites in her system? This is important.
I agree that milk could be the culprit. Many kids have a hard time digesting milk products after a stomach bug. There are other more serious problems it could be if the symptoms persist. Celiac Disease is one that fits the description. If the problem continues, you should consider having her evaluated by a gastrointerologist.