I have a new co-worker who is joyous, bordering on annoying. I rather liked them because it was nice to hear such a cheery person in a mundane environment.
We went to break and collected in the breakroom to relax for a short time. There was a potluck set out for people to enjoy. The new person sees me sitting alone and enjoying the snack I brought. She asked why I didn't have anything from the potluck. I said I didn't want any and thank you. She asked why. I pointed to the sign that stated, "Shift potluck is for contributors only." I said I hadn't brought anything. She said that's okay and I should get something anyway. Now, I'm type 1 diabetic (have been for 30 years) and explained that I bring my snacks because I keep tight control on my calorie intake. She then wandered off to the potluck table and returned with a plate of food she grabbed for me. I let it sit there.
In addition to the above reasons, I don't like potlucks because I'm not sure how the food was stored, handled, and/or how clean my co-workers hands are. Some people find it weird but I'm susceptible to illness (considering I have an autoimmune disease) and illness is generally more intense for me. I didn't tell her this because it's not her business.
A few minutes later, she comes back and asked if I was going to eat the food. I said I didn't want it and that I wouldn't but also didn't want it to go to waste. She took the plate and returned to her seat.
It should have been 'I didn't want any' and nothing more. I don't know how I could've been clearer.
Edit: a lot of people are missing the point that my first response was "I don't want any thanks."