I would say to anyone who asks if you are handicapped/disabled the following:

"Why do you ask?"

I only say this because I think if you get an idea of why people keep asking you that question, there may be something you can do to change people's perceptions of you and I mean that in a good way. Or, maybe you'll just find out that people are rude sometimes and it would make it easier for you to ignore them.

IF they get more rude with you, the next best thing to say to them as far as I'm concerned is:

"Your opinion of me is none of my business."

I'm sorry this has been happening to you.