@boomerang,
Is it possible that you got stung by a bee on this hand too? It doesn't sound like it would be from holding a paintbrush.
Do you have any tingling in your fingers? Go to a doc right away if yes to tingling.
Is the swelling spreading? Did it always reach up into the wrist or did it start out smaller? Go to a doc right away if the swelling is spreading.
Have you finished the project? Go to a doc if the activity has stopped and the swelling hasn't started going down.
Is the swelling from the bee sting on the other hand more extensive than you'd expect from a bee sting (localized swelling at the sting site but not more than approx quarter-size in diameter)?
Do you get big golf ball to baseball sized welts from stings? If so then this could be a reaction to a sting that you don't remember getting.
Is there any discoloration/bruising?
I think you should get it checked out by a doc. Particularly if you've got a bunch more painting to do.