28 month old throwing up with no other symptom?

I just got off the phone with the doctors office after waiting 25 minutes on hold. My 28 month old daughter Rylee has vomited 4 times in the past hour. She is acting 100% fine, until she throws up. She just stops walking and projectile vomits everywhere. My 1 year old threw up once a couple hours ago, but is fine now. Rylee does not have a fever but keeps telling me that her "belly hurts" but is playing fine. I am trying to make her take it easy and lay down for a while. So the doctor really didnt tell me what was wrong with her, just that she wanted to see her tomorrow if she doesnt stop throwing up. So is this stomach bug or what?