Hello,
I have gone through your symptoms and history and following is my answer:
Since you have felt this lump from 4-5 years now, one of the most common reason is a
lipoma (accumulation of fat tissue).
It correlates with your symptoms, it gets bigger with the passing of time, doesn't hurt, soft to touch. The discomfort that you've described might be due to the contact to the nerves near it and the veins of the armpit.
I'd suggest you do an
ultrasound of this lump and consult a surgeon. If needed s/he will ask for a fine needle
biopsy for the diagnose and do a surgery to remove the lipoma.
Don't worry, if this is the diagnosis, it is a benign condition and it can be removed easily.
Hope I have answered your query. Let me know if I can assist you further.
Take care
Regards,
Dr Ivan R. Rommstein, General Surgeon