For me, it was the introduction of other aspects of the role which I knew would up the secs already quite tricky workload. I took her off for a meeting, we had a long chat, made joint plans and agreed that we were going to review weekly in the first instance and see how she feels it all went after the first month. This is our completely informal arrangment. I write notes to keep track of how we're doing and what we've agreed.
I think for a more formal thing where the manager says they have concerns for your workload then it's the same meeting but specific areas where things are not rosey, discussion with you re what you think the problems are or if you are even aware of a problem, a work plan is agreed with set time scales (and regularly reviewed) with set goals to get things back up to standard, if it all goes tits up and then stage one disciplinaries start (I think it's stage one, might be two).
The latter doesn't apply to you anyway as it's you calling the meeting and asking for a review of your workload.