Did this appear suddenly? I really can’t see well on the picture, but it looks somewhat like a puncture o me. But then I would expect a puncture wound to be painful. I think that this should get looked over by a vet.
Yes, that does need to be seen by a vet. Did you put medication on it? If not, then it is certainly infected. I can't decide if I agree with abyeb
, that it looks like a puncture, or if I think it looks like a bite from a venomous spider or some insect. Either way, it should be seen, and soon.