I just wanted to add to this, since it happened to me and my dog. Unfortunately, I did NOT read/find this thread at the time
Anyway, swollen eyelid, just like the picture. I treated several times a day with warm washcloth/compress, also vet prescribed doggie Benadryl (called something else. Little yellow pills) for about a week, as I thought it was a stye. In reality, it was a spider bite. And actually, there were like, 8 of them on various parts of his body. Some dime/quarter sized that became infected/bursting abscesses. One MONSTER one along his neck that had filled with pus and required emergency surgery, a drain in his face, antibiotics, doggy narcotics, the cone of shame... And all told, about $800 in vet bills. AND 3 weeks of vet visits, constant care, and twice daily home first aid.
Upside: The benadryl saved his life - It was, according to my vet, an allergic reaction that could've killed him. (He's just a little guy.) Downside: Had I gone to the vet sooner? I could have saved my dog and myself a lot of heartache, pain, and money!
I'm a VERY involved dog Mommy! To the point that my friends laugh, honestly
But my dog had never been sick - I didn't know what to look for. So this is what I'd look for if it ever happened again: The swelling. If it doesn't go down in 24 hrs, TAKE DOG TO VET. Check his whole body for more unexplained bumps. If you find bumps, look beneath the fur for a red/black/purple ring around the edges (sign of necrotic skin). Displacement behavior!! At the time, my dog was licking his male parts like, 100 times more often than usual!! I've since learned that was to distract himself from the pain. (Which makes me CRY inside that I didn't know!!)
Lastly, get pet insurance. I bought a mid range plan for like $25/month that covers everything - even more bug bites after 6 months- So I NEVER have to go through this again!