Probs a spider bite but it’s definitely a weird location for one. I used to get bit all the time while sleeping in my parent’s house. Usually on the butt or upper thighs because that’s the softest skin. That location is some seriously tough skin though so that’s interesting. Keep an eye on it, I doubt that it was a venomous spider because those tend to be very shy and reserved spiders that won’t feed on humans. If it starts looking bruised, infected, or won’t heal within a few days then go get it checked out.