if you have the ability to go see a doc, don't try a self diagnosis. period.
and then don't always trust the first doc you see.
i've been bitten by lots of stuff over the years, and have gotten infections from a few. my docs often guessed spiders, i suppose, from my suggestion; not from any kind of testing.
one time a doc said i had a spider bite and treated it as such, but a second doc ended up cutting out the head and claws of a wood tick a few painful days later.
from conversations with many appalachian trail hikers i've met, doctors seem to gravitate towards spider bites for explanations, but i tend to doubt that they all were. there's lots of creepy crawlers that have a taste for human blood.
just for scare value, the recent outbreaks of super resistant staph infections, m.s.r.a., was misdiagnosed as spider bites for a long time. here's some info on m.s.r.a. (please read the healthcare provider info): MRSA Infection (False Spider Bite)