Could you have a mosquito bite on it?

Once I had a bite on a knuckle and it swelled up and turned red.

Try some ice on it, see what happens.

I've had various finger "attacks" which typically for me come from
carrying groceries in those plastic sacks, when they are too heavy.

Did you get a Strep test yet? Strep infections can show up elsewhere in the body, as reactive arthritis or skin rashes.
Scarlet fever is one name for this.

So if the finger continues, you should see a doctor.