Plegm vomiting

My daughter is almost 2 years old. From past 1year she has been occasionally vomiting phlegm. Its usually after her meal and sometimes in her sleep. The frequency of vomiting is once in 2-3 weeks. The vomit consists of the food that she would have eaten, but it is accompanied with a lot of phlegm. She never complains of any uneasiness before or after vomiting, but she definitely looks a lot relieved  and  sleeps better after vomiting.Her doctor says its normal. But I am still concerned. It would be of great help if you could tell me what causes this problem and what can I do to prevent it.
Thank You,
Latha