"Are You Okay?"

It's a simple question, although the answer doesn't always seem as easy. That question and others like it, have been posed to me lately and they have kind of caught me off guard. I don't really know how to answer it. Here is what goes through my head when I hear that question:

1. I feel fine, but now that they are asking, I wonder if there is something wrong and I don't look okay.
2. I have been quieter (thought I would try something new at work- stay to myself and not get into other people's business), others are interpreting it to feeling ill or not being "O.K.", is that why they are asking.
3. I am tired (not enough sleep), but today day I feel just fine, is that why I am getting asked the question.
4. Should I just make something up and go with it.
5. Do I tell them I'm fine and then watch them get a very concerned look on their face, like I am just saying that to get them off my back.
6. Why are they asking me that? Do they really think something is wrong?

Just like I said, it is a simple question, but the answer isn't always as easy as it seems.