Very VERY few "Spider Bites" are really spider bites. Although in your instance, it could well have been. There are several species which are capable of biting, and they all do it defensively--- not like many insects which bite to feed on blood. SO, spider in your waders assumes it is being attacked by weird pressure and being trapped between your leg and the fabric, so it bites... Thankfully not a widow or recluse. Could also have been a small wasp, ants, or any number of other things.

I got bit by one of those cute little black/gray and white jumping spiders which we always see on walls and rocks... there was one on our window curtains at home... I thought I'd just grab it and put it outside- wrong. I grabbed it and got a powerful bite right between my thumb and index finger! Felt like a wasp sting. the pain only lasted a few minutes, but it was memorable.