Sounds like you have a few concerns. I am not an expert in demon possession, but I believe it would be something very obvious that you yourself would not have control over. You would not ask because the demon would not allow you to expose him. That being said it doesn't mean one cannot be tempted by a demon in serious way, but that's not possession.
How do you get sick from Communion? Assuming it's not just your impression, I would have a close look at this phenomenon. I can imagine many things, but since I don't know your own situation, I can't really say anything.