Sounds to me like ingrown hairs. I'm not an expert, but that's what it sounds like. I would imagine whats happening is that on some of the hairs you are tweezing are breaking below the surface of the skin causing irritation. When the hair breaks below the surface, the hair can grow sideways or curl.
A tip would be to tweeze as close to the skin as possible.
Here are some tips for tweezing.
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1. Brush hair in the direction of hair growth.
2. Isolate the hair you are about to tweeze.
3. Tweeze in the direction of hair growth.
4. Pull one hair at a time.
5. Pull gently and smoothly, making sure not to yank.
I would imagine steps 3-5 are the most important.
Just my thoughts.