This sounds like it could be a blood clot and could be life threatening. If there is an emergency clinic nearby (or even a distance away), I would get your cat evaluated immediately. This cannot wait for 3 weeks, and I would be calling anyone willing to see my cat. There is also a possibility that it could be a bite or a sting and this is an allergic reaction. If you are absolutely unable to contact anyone, I would get some chlortrimaton 4 mg tablets from the pharmacy and give your cat 1/2 of one. If this is a sting, or bite, this may help to keep him from going into respiratory distress until you can get him into a vet. He definitely needs to be seen today, as either scenario could be very bad.
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Lori McLaughlin
Your veterinarian should ALWAYS be consulted first and foremost when your pet's welfare is at stake.