it may not have ever been a spider bite. one consideration would be a herpetic whitlow -- a fever blister/cold sore of sorts that occur on the finger. it will come back over and over itch at first and will definitely get red or even have a tiny blister. like most herpetic infections the body keeps it "suppressed" most of the time unless of course you are going through an illness or stress! look this up on the internet and see if it fits. as you may know i cannot make a blind diagnosis over the internet but your story seems to point in this direction.
i got bitten by a spider or something like it years ago on my thumb. redness and itching keeps coming up. its been years since this bite on my thumb and it went away at first but months later i noticed the same place was swollen and itchy. it goes away and comes back months and months later. i've noticed it happens when my body is in stress or im going through sickness. i dont understand why this same site keeps getting swollen and red. its been years. ive asked my doctor about it once and he said oh it'll go away on its own. this doesn't seem to be happening.