What do flight attendants do if a passenger asks what is going on in case of a potential emergency? This is related to, but not the same as, another question I asked on Interpersonal Skills.

I was on a flight last year, on a route I fly on a fairly frequent basis. Shortly after takeoff, I heard the three double chime signal, which is commonly used by airlines to indicate to the crew of a potential problem. The crew quickly marched up to the front of the passenger cabin and was briefed by the pilots. I also noticed that the plane was going off course. Some time later, I used the onboard Wi-Fi service to query an online flight tracker about the flight I was on, and it said we were diverting back to our origin airport.

Throughout the whole incident, all the crew kept quiet about the whole thing, and didn't let passengers know about the diversion until just a few minutes before landing back at the origin airport.

Before the announcement, while the seat belt sign was turned off and while the crew weren't making any emergency preparations of any sort (just casually chatting with other passengers), I got up and asked the crew what was going on, and they said "nothing". I then showed them the tracker page which indicated the diversion, and they just asked me to return to my seat.

What's the general protocol for cabin crew, when asked by passengers who notice potential issues like this and get curious? Is it common for them to simply say that "nothing" is going on until the flight deck makes an announcement? What if a passenger really insists on knowing what's going on? (As an aviation enthusiast, I feel more comfortable if told about the issue; not knowing about it can make me feel tensed.)
What's the general protocol for cabin crew, when asked by passengers
  who notice potential issues like this and get curious?


The flight crew and pilots are under no obligation to inform the passengers about non critical issues and broadly speaking it's often better to provide only needed information so as not cause some kind of panic. In the vast majority of situations on an aircraft there is nothing the passenger can do other than evacuate when told to do so.  

Turning an aircraft around and heading back to the point of origin is not necessarily an emergency although it can be there is nothing to say it is or is not from a passenger point of view. 

Even if a passenger insists the cabin crew will likely ask them to take their seats, buckle up and await any further instructions. It's possible that the cabin crew is not even informed of anything as the pilots try and work the problem. 



The later part of your question is answered here and the general answer is, unless you are a trained pilot it's hard to actually know if something is going wrong. There is a very interesting example of this from the Air Florida Flight 90 incident which is the only incident I know of where a non flying pilot spotted an issue and took action, this would be a fairly rare case; 


  Survivors of the crash indicated the trip over the runway was extremely rough, with
  survivor Joe Stiley – a businessman and private pilot – saying that he
  believed that they would not get airborne and would "fall off the end
  of the runway". When the plane became airborne, Stiley told his
  co-worker (and survivor) Nikki Felch to assume the crash position,
  with some nearby passengers following their example.