Sudden S card failure after 7 months iPhone 4S

I've had my 4S (not jailbroken) since December, and haven't really experienced any problems with it. Occasionally the phone would drop cellular data coverage for no apparent reason, but turning it off and on again usually cleared that problem up.

Suddenly, this morning I got a SIM card failure (Invalid SIM). This happened when I was driving to work with my phone plugged in for charging (something I do frequently). When I got in to work I removed the SIM card and re-inserted, which seemed to fix the problem. However, on my drive home from work the same thing happened (again while plugged in for charging). Again I re-inserted the SIM, and then plugged the phone in to the computer. Again, after charging for a few minutes I received the Invalid SIM error.

The latest thing I've tried is resetting the network settings, which seems to be working for the moment, but I'm afraid this is going to be an ongoing issue. Is it possible that the SIM has suddenly gone bad after working fine for 7 months? Should I just go to AT&T and ask for a new SIM or is this more likely a hardware problem and I should just go straight to Apple? I have AppleCare on the phone, so I'm not concerned about costs, just don't want to be making unnecessary trips.

I'm not sure why this would start happening all of a sudden. I'm running 5.1.1, but I did that update back in May, and everything was fine. The only other unusual thing is that I was traveling (within the US) over the weekend, so I did switch in and out of Airplane mode a few times, but that has never been an issue in the past.