It's crucial to take your cat to the vet immediately. The bumps could be a number of things, some serious and some not. A vet can properly diagnose the issue and recommend treatment.
Here are some possible causes of bumps under a cat's armpits:
* Allergic reaction: Fleas, food allergies, or environmental allergens can cause skin irritation and bumps.
* Fungal infection: Ringworm is a common fungal infection that can cause circular, scaly patches.
* Bacterial infection: Staph or other bacteria can cause inflammation and pus-filled bumps.
* Mites: Mites burrow into the skin and can cause intense itching and bumps.
* Abscesses: These are pockets of pus that can form under the skin.
* Tumors: While less common, tumors can sometimes appear as bumps.
Please remember: Only a veterinarian can diagnose and treat your cat's condition. Don't delay seeking professional help.