What Causes Painless Swelling Under Right Armpit?

Hello; I wonder if you can help me. Some 2 weeks ago I noticed that it was just a little tender under my right armpit. Then maybe a tiny swelling. Today it seems to have swollen a little more, but more solid. If I touch it I am aware of it, but otherwise no pain. I am female, living in the UK and 53 years of age. 3 years ago my thyroid swelled up really quickly. I had it tested; all is fine; it's hereditary. I am currently under stress, but otherwise very healthy; only been to hospital to have my daughter. Many thanks indeed