Very mild cough making 23 month old vomit at night?

[Feenie]

My ds has the mildest cough ever, but for 4 nights in a row, about twice a night, he has coughed once which makes him vomit all over the bed.
I can't believe such a little cough can make him do this.
He is absolutely fine afterwards and during the day, and is eating normally. Should I be worried, or have others got children who vomit so easily?

My DD vomits quite easily die to coughing but she does have a rather hacking cough!
We try to raise her head height in bed when it happens as it's the build up of mucus on her chest that then is swallowed and causes her to be sick.

DS1 tends to be sick if he coughs alot he doesn't seem to effectivley clear phlegm from his throat and it makes him gag and then vomit (sorry if TMI)
He has had a cough since 10 October (sad that I know the date!)but apparently we just have to sit it out.