That looks like an ingrown hair to me. Bella came in from my hubby letting her out one morning last week and jumped up in bed with me I took one look at her and freaked. Poor thing she was scratching her head with her paws because she had pea sized welts all over her head and neck, her eye was swelled almost shut and her muzzle was twice it's size. She looked like a boxer and she's a Greyhound! The vet said it looked like a wasp bite. That all happened within a couple of hours after being outside. If your baby had that bump for a while it's probably an ingrown, although, not quite sure what a spider bite would do. Good thing you're seeing a vet for it. Bella and Zayla send lots of healing paw prayers.