The first thing that comes to mind is that you could have a staph infection. If that's what it is, it can be quite serious and it won't get better if...
We eventually got completely off soft drinks, but I used to make my own versions at home. I bought root beer extract and added that and simple syrup...
You don't really need an excuse. Saying "no thank you" with no further explanation should be sufficient, and if someone pesters you after that, then...