When my DD's dad changed jobs I didn't get any child support for 3 months. When I called them they said it takes up to 3 months to get a new collection order in place for their new job.
Also, if you ask for a change of assessment because circumstances has changed, there is a hell of a lot of paperwork to fill in - but it's certainly worth doing in the end if it changed the amount you got.
I'd call them ASAP and enquire about what you would need to do, that way you can get the paperwork and start the process incase you need because it's a long process sometimes!