I had something very similar to that this last week. It reacted and it swelled into a hard area about the size of a ten pence. It was very itchy and hard to stop scratching. It sort of turned into a scabby top, and wept a bit.
The itching has gone now and I’m left with a small scab which is diminishing. I’ve had this sort of thing before and I reckon it’s just a bite of some kind, probably a horse, or a fly, or a horse fly.
It is my professional opinion, as a cabinetmaker that you have nothing to worry about.