some tricks to look sick when you indeed are feeling terrible:

Don't dress up for the doctor. Don't put make up on if you are female. Making yourself look wonderful undercuts your believability. If you have to work, just cover the bases, nothing fancy.

Limp some now and then. I find a groan, huff or puff placed properly and not overused, very effective, especially in stores.
Move slowly, and carefully...to avoid trips and falls.

Have more bad hair days than usual. Cover hair with hats, caps, scarves.

If you look in pain, people often believe it...and you don't have to remind them. A well placed groan, in a store is very effective for me!