My 4-year-old Siamese cat's behavior has changed very abruptly starting
yesterday morning.  Usually, she is quiet, playful and likes to be around me.
But since yesterday she constantly licks herself (especially her back, her
belly and her paws), scratches herself very vigorously and twitches her ears or
shakes her paws as if trying to throw something off.  She does this for a
while, then suddenly starts running, stops, and does the same thing again.
Eventually she will quiet down but after a few minutes the same behavior starts
again. She hides herself in dark places, something she has rarely done before.
Her eating patterns haven't changed, but she doesn't play anymore and doesn't
meow.

I took her to the vet, who checked her for fleas and mites but didn't find any.
After checking her ears and combing her fur with a fine comb, the vet told me
that this has probably something to do with allergies or stress. The vet made
an injection of some medicine that is supposed to quiet the cat down.  

When I brought the cat home, her licking and scratching continued.  So now I
don't know what to do.  Could this behavior be the result of stress from the
noise outside our building? For a few weeks now some repair work is being done
right outside our windows with workers using jackhammers for hours, etc.  Or is
it the dust from cement that blows into the apartment through the window
because of the repair work?  Or maybe her sleeping on the cable box has
something to do with this? I mean, static electricity?

What should I do next?