My son is 18 months old, and started throwing up out of the blue. his throw up is has a lot of mucus. before he throws up, we can hear the phlegm. he does not have a fever. what could this be?

Reflux: It may be reflux, and hard to say without more information. I would check to see if he has anything new in his diet that could be triggering. Does he have other symptoms: cough, runny nose, fever? Those could indicate that he has a viral gastroenteritis that will run its course in due time. Be sure to keep him adequately hydrated.

Vomiting:: When a young child has phlegm and mucous, often from a viral illness like a cold, they may throw up the phlegm to help clear it. An older child or adult can cough up phlegm and spit it out but a younger child like this will tend to throw it up. Vomiting can also be a separate symptom of an illness. Make sure he is staying hydrated and monitor his breathing. Contact your doctor if worsening.