Is this a panic attack, or something else?

it could be. the most important thing to remember is that when you feel this way DO NOT RUN AWAY! you will never get over your fear if you run away from every social situation. Next time it happens, stop, take a deep breath and try to think about what is making you uncomfortable. Is it really the fact that you think people might be judging you? (don't answer that right now, wait till you test it in a group of people) You might think that is the answer, but there might be some underlying issue that may come up that you will have to deal with to overcome this. A good tip is that the more times you FORCE yourself into a social situation that you find uncomfortable, to faster you will overcome your fear.