I have had a lump in my armpit since age 39. I am now 54 yes of age. I was out on a blood thinner at the time, and  had a battery of blood tests done. The good news is  that they found nothing wrong with my blood. I do however take a baby aspirin daily as recommended by by Doctor.   I sometimes wonder however if there is a risk of the clot travelling to my lungs or heart.
Welcome to Chat Doctor .1. The history suggest that it could be a lipoma (fat cell content mass lesion) because long-standing and pain less, no significant growth. 2. Next possible cause could be cystic structure, also seen pain less and may remain for longer time, finally lymph nodal pathology can not be ruled out.3. In this case I would recommend an ultrasound scan to evaluate the sours of lesion / lump and its nature (cystic, solid, vascular) and where all its extending can be assessed. 4. There is a risk probably - so your treating doctor has kept you on the aspirin on daily basis.5. So when you visit doctor next time try to get the scan done. An FNAC - will help to know the cells of the lesion (fine needle aspiration cytology) Good luck. Hope it helps you. Wish you a good health. Anything to ask ? Do not hesitate. Thank you.