my 18 month old son has asthma but also keeps getting chest infections, at the moment he has a nasty cough which he is coughing up a froth like liquid (smells a little like vomit but doesnt look like it) what could it be? he cant sleep,hes crying when he coughs, his appetite poor and several dirty nappies a day. he had bronchitis at 3 month old and a chest xray last week wqhich came back clear, im confused with the clear chest but the constant cough