Thing is with synthetic carpet is its inert, so wont obsorb as much prespray, as say a wool. This means its alot easier to overwet a synthetic carpet as the prespray, if over done, will go straight down to the backing.
I'm not saying you did over wet, but you could have. but its most likely there was alreadly mildew in the carpet, soon as it got wet it activates the smell.
Solution is simple, Prochem microsan, spray on leave to dwell 15 mins extract, it will kill any odour causing mould/mildew/bacteria.
student basement flats use it anyhow because your almost certain to get this problem in these damp/dirty places, saves coming back.