Strange sickness but not sick...can anyone shed some light on this???

[Dozymare]

DS2 is 3 and 6 months. Last night, he had a nightmare, came into bed with us, and around 3:30am he woke me up with the sound of him vomiting. However, on closer inspection, this wasn't vomit but a large amount of saliva...He quite literally sounded as if he was choking on his saliva....We changed the bed, he went back to sleep and again this morning, he was retching in the loo - lots of yellowy saliva but not sick as has no odour IYSWIM.

Any ideas as to what this could be?

Has he had a cough or cold recently? DS 3.10 threw up a couple of weeks ago, 6 x in the space of 6 hours, all white and yellow phlegm stuff. He'd had a cold for ages before hand though and was told it was just all the crap off his chest.

thanks ladies - he seems much better now - hasn't obviously had a cough or cold although his sister has been under the weather...thanks for responding!