Depending on who is doing the asking, you can tailor your answer to what you feel comfortable telling people given the situation. For example, you might tell them you consult (in many different areas); that you're independently wealthy

; or that you work for the Lord

. Alternatively, you might just simply state that it's really not worth getting into, or that truly, it's none of his/her business!! Then, in that uncomfortable second, quickly deflect the attention to them by adding, "And, WHY do you ask?" AGAIN, it all depends on your personality, on who's asking you, for what purpose, and what you're comfortable saying. Obviously, if it's a valid reason like you're taking out a loan at the bank, you simply say "I'm disabled, and I receive $X monthly in SSD or private disability insurance." You don't need to state the reason for your disability.
However, NEVER be embarrassed about who you are, or what you do, or are no longer able to do for a living. If you're disabled and unable to work, it's really between you and your doctor about your health. I find that in most cases, the person doing the asking is an outright stranger, and really are not interested in YOU, or genuinely a concerned philanthropist looking to help you pay your bills!! Usually, I find that the person asking is just rude and being nosy.... Especially, if you don't know or just met the person, and are unlikely to see them again... It's just them demonstrating poor manners.
I don't usually get asked about working or my source of income. Instead, I occasionally get the "Oh, did you hurt/break your leg?" question because I use a cane. It's the only real outward sign of my disability that is openly visible to others. To those people, say the cashier at the gas station, I sometimes just sort of nod vaguely in agreement with them... Or, I will shock and silence them by telling the real reason why I use a cane, saying "No, Advanced Cancer." This usually seems to do the trick since I'm still a relatively young guy. They don't know what to say, and you can kind of hear, in that moment of awkward silence for them, that they were sorry they asked you!!
Take Care and Good Luck,
CancerDad