Weird indent on my index finger, came out of nowhere

So I went back to work recently and have been handling PCBs (process circuit boards). I do not play the guitar or anything like that. I have not handled many things for months and I suddenly got a slightly sore, rough-feeling deep indent on my left index finger. First time I see this, never had it before.

Does anyone have an idea what it is? It is not extremely painful. It gets sore only depending how I press on it.