When my DD2 was a baby she was sick quite often, to the extent it didn't feel like normal bugs. I went to the GP they ruled out anything sinister & put it down to bugs.
As she got older she started to say her head hurt & she'd often be sick afterwards, then feel better after rest, Calpol & 20-30mins. This has gone on for 10yrs.
It's generally when she's too hot / exercising & her headaches are more migraines (though GP said not, but I have migraines & know what she's experiencing is more than a headache).
If she doesn't immediately cool, stop to rest, take painkillers etc then she will be sick.
She drinks plenty of water - fluid diaries done, but it doesn't stop it happening.
It's really hard to know what to think when they are little as it's hard for them to explain. Hope you get some advice from your GP.
On another note, DS1 was also sick all the time (multiple times a day, more catarrh than sick) turned out to be a dairy intolerance.
You could try removing dairy for a couple of days / weeks see if it helps.
DS1 grew out of it by reception (also had asthma & alopecia - all auto immune - grew out of asthma & his hair grew & thickened).