Without knowing all the details, this sounds like a TCAS RA which has been explained by other posts on here. If it was a 'nuisance' advisory (as your pilot said they were unsure if another aircraft had levelled) this is usually caused by an aircraft climbing or descending very rapidly to the level above or below another aircraft (1000' is the standard separation). The TCAS sees the rapid closure, and as it's independant of both the autopilot and the pilot's minds, doesn't know that the climb or descent will shortly be stopped at a safe level. For this reason, crews are encouraged not to make very rapid climb or descent when approaching their cleared level. A TCAS RA will result in paperwork being filled in, probably from both crew and ATC, but as long as separation was never lost or likely to have been lost, it's unlikely you'll ever get to see the details. Think of this situation like a go-around just before landing: it's an uncomfortable feeling and it's very unusual....but it's a standard aviation manoeuvre, practised by the crews on a regular basis, and is designed to ensure safety.