Is this reaction to an injection today?

Question
I have two siamese cats.  One male (3 years old) and one female.(4 years old) They are spayed and neutered.
I took the female to the vet today with a running nose.  Vet said she had an infection.  Gave her a shot of clavamox.  The minute I walked back in the house with her she started hissing and fighting with the male cat.  They have never fought before and have always gotten along.  Tonight she seems lathargic and his moaning.  Have any ideas or advice for me?
Thanking you in advance.

Answer
Hello.  In my opinion, the lethargy is probably related to the infection rather than the injection.  Clavamox may cause loss of appetite, vomiting, and diarrhea, but it does not usually cause lethargy.

Injectible Clavamox is given intramuscularly, and this causes the injection site to be quite sore.  So some irritability doesn't surprise me.  When you add the soreness of the injection, the discomfort of the infection, and the stress of a vet visit, I can certainly see why your female is reacting aggressively toward the male.  It's natural for some cats to lash out when they are feeling uncomfortable.

I'm sorry it took me so long to answer.  Allexperts.com must have been having technical difficulties, as I couldn't get to the answer page.  I'm hoping by now you have seen improvement in both her health and her mood.  If not, give the vet a call and let him or her know she hasn't gotten any better.  You should see a difference within 24-48 hours of the injection.  And if your two are still bickering, keep them in separate rooms until your female is back to full health, and then reintroduce them slowly.