A lump under the armpit could be a Lipoma (consists fat), Lymph Node Swelling, or Tendonitis. There could also be some rarer reasons like sebaceous cyst or dermoid cyst.
Need not worry about breast cancer at this age. It commonly happens after 40 years. A lump only in the armpits and no detectable lumps in the breast is unlikely to be breast cancer.
Next Steps
Do visit your GP for examination of the lump. You can also request for a Clinical Breast Exam to allay some of the fears.
Health Tips
Avoid relying solely on Google/ChatGPT for diagnosis and treatment as this could delay the actual management.