This is Dr. Christian. Welcome to JustAnswer. How long has the swelling been there and is it getting larger? Is he eating, drinking and otherwise acting normal? Can you see any wounds?
This is a stray cat that we have been feeding.
Today I noticed that there is a huge lump under the "armpit" of the cat's front left leg
Don't know if it's been there long
He (un-neutered male) has been eating fine
wet and dry food.
At this age the most likely cause for a swelling like this is due to an abscess. An abscess develops 3-5 days after a bite wound and is a pocket of infection under the skin. It can make a cat feel pretty sick as it gets larger and the infection worsens. Once the abscess is opened, drained and antibiotics started it will heal quickly in most cases. There really isn't anything to be done at home under the circumstances, your best option is to have him seen by your vet.
Thank you for your advise. It doesn't seem to bother him when I touch it. But I will definitly see if I can get him to a vet.
You are welcome. Good luck.