Could be a fluid build up called vesicles on a vessel. Should drain over time. Like a small abscess from a draining vessel or insect bite. Anything.
It could be an insect bite or maybe a blocked lymph node if it is at the back of the ear . Probably best to have a vet have a look at it .
Is your dog in any pain do you know? Or scratching/licking it's ear? Call a vet. Make sure everything is okay. Could be cancer, or maybe just a bite from something. But make sure to call.