Hi Alwyn,
Is sounds weird, usually infectious diseases causes a rash behind the ear and in the palm of their hands and the soles of their feet. (I know 'cause my Paed told us that when I took my LG in for weird spots that turned out to be mozzie bites)
If this only happens after a bottle and disappears, there might be a milk allergy, they can never remove the cow's milk out of formula completely, except with the HA formulas (Like NAN HA), where it has been removed due to repetitive processes.
If it goes away, it can't be due to creams or washing powder. I'd really suggest you go see your paed on this as he / she will have the experience to advise you. It just sounds very much like an allergy to me,