There is a bite by my balls maybe a spider bite?!?!?!?!?

Last night I slept on a little mattress on the floor. I was fully clothed and had a pretty heavy blanket on. This morning I woke up and had to itch down there real badly. Now there is a small red bump. I don't think there are many poisonous spiders around here. I mean how would a spider get through the blanket, on the bed, and inside my underwear, just to bite me! I mean it could have been maybe my arm, but why there! So I read that a spider bite can cause skin to rot. Maybe I should just put some Benedryl on it and keep an eye on it?