It may be a couple things. One is, though it has been a bit of time, is a hematoma from where they drew the blood. Another is a swollen lymph node. There's alsothe chance that they did give fluids under his skin and didn't tell you and the fluids are absorbing slowly. They will disperse as they are being absorbed but sometimes, from the weight of the fluids under the skin..gravity will make them shift. There's also the chance that it has nothing to do with the vet visit and he got a spider or bug bite that was making him feel off to begin with and has now abscessed. At any rate, a vet visit is needed to check itout and determine what it is.
Let me know if you have any other questions
Good Luck &
Best Fishes