I've been reading a few sites...

Congestion or a respiratory infection can also lead to vomiting, since swallowing excess phlegm and mucus can irritate your child's stomach. (from babycenter.com)

Allergies - if he/she's had an allergic reaction to something (a holiday plant, a decoration or scent that's being used, etc) This is just from my observations of my own situation, my daughter has had an 'unrelated' eye infection for over a week and it hasn't gone away w/antibiotic treatment.

Something is lodged in her throat (my thoughts have turned to something related to the Xmas tree - maybe she ate a 'leaf' from the pine tree and it's gotten lodged in her throat or nasal passages, etc) - also a fear that crossed my mind due to how fast it seemed to come on.

Poisonous substance - Your child could be vomiting if he has swallowed some kind of drug or toxic plant, herb, or chemical. What to do if you suspect he's swallowed something toxic, call the American Association of Poison Control Centers' national emergency hotline at (800) 222-1222, immediately. (also from babycenter.com)

Here is yet one more link discussing the same issue - another au site: http://members.essentialbaby.com.au/...&#entry1898052

Good luck, and again, I will write more if I find anything else out.