Many people experience this. The interview goes well, the CO says you are approved and then within a day or two the CEAC system shows the status has been updated to AP (Administrative Processing). So – that means one of two things is happening.
The usual thing is that the CEAC is updated to AP during the finalization of the processing. That typically lasts one or two days (work days, not counting weekends), and then the status is updated to “Issued”. For the majority of cases this is what happens, it is nothing to worry about. Some people never even notice the brief period in AP, and some people just see the status stay on ready for days after the interview, then it is updated to Issued OR the passport is delivered. None of that is a problem!
The second case is where a case is “approved” in the interview and then the case needs some time in “real” AP. This can happen when the supervisors at the embassy review the cases approved by the CO and decide a case needs some checks. That type of AP can last days, weeks, even months – and is obviously very worrying for the selectee. So – if you case stays in AP after a successful interview for several days (let’s say a week), then the second type of AP has started. There is NOTHING you can do about that – you just have to wait.