4 year old with vomiting, no other symptoms

[albysmom]

My 4 year old daughter vomited about 2 weeks ago in our car. She had fallen asleep and we had been driving for about an hour and got stuck in traffic. She woke up and was coloring and singing, very happy. We started moving and got off on our exit about 30 minutes later. She then vomitted twice. She cried because it was all over her. I changed her into clothes that I had just bought that were in the trunk and cleaned up the car. She was perfectly fine after that. Happy as could be, no fever, no loss of appetite. I thought maybe car sickness (new car) or something she ate. Well, 10 days later she came into our room at 5am. She didn't say anything so I got up with her and she started to gag. I got her to the bathroom and she threw up. She then was fine, no fever, no complaints and I put her back to bed smiling. I got her up at 7am and she still fine (actually didn't even remember doing it). I took her to the doctor and he said he could feel that she was backed up/constipated and to increase her miralax. She has had constipation problems in the past and been on miralax for years. My concern is that it is something more serious. Call it mother's instinct or just worry from past experience. A couple of years ago we had a little neighbor girl who was vomiting on and off for a couple weeks with no other symptoms. Was told it was a virus, then ear infection, then GI infection. It ended up being a brain tumor and she was in heaven 3 short days later. I can't get this out of my mind and pray that the vomiting stops soon. Is vomiting a common symptom of constipation? I just can't believe that she would suddenly start vomiting from a problem she has had for years now. Please advise. THANK YOU.