I don’t want to frighten you but, in my view, you should never treat lumps under your arm lightly.
Several years ago, I had a soft lump under my left arm. My GP sent me to a consultant, who told me it was probably nothing, but sent me to have an ultrasound. The guy who carried out the ultrasound was very unpleasant and told me it was a fatty lump and I was wasting his valuable time. He was almost certainly right, but his unpleasantness put me at risk.
In 2015, I went for a routine mammogram and was diagnosed with a slow growing cancer in the right boob. I had a lumpectomy and got on with my life. Within a few months, I noticed that the lump under my left arm seemed to be getting bigger. I assumed that was because I was getting fatter and I didn’t want to waste anyone’s time.
However, because of my breast cancer, I receive annual mammograms and in 2016, precisely one year after my first diagnosis, I was diagnosed with a brand spanking new more aggressive cancer in the left boob which had spread to the lymph nodes under my arm. I think the doctors thought I’d had it as the lump under my arm was as large as the one in my boob. However, one very valiant little lymph node had kept all the cancer cells within it, for which I am very grateful.
The moral of the story is always get lumps and bumps looked at. You are most likely right and it is nothing to be concerned about, but better to irritate a member of the medical profession than to take unnecessary risks. My best wishes are winging there way over to you.