Thanks for your patience.
I can understand your worry.
From the information that you provided , it seems to be a lymph node. There is nothing to worry about. These can be felt even when there is a slight inflammation/ infection in the underarms or around that area. They usually resolve on their own. there is nothing to worry, they are felt many times without any serious pathology.
I would suggest not to worry about it, but to treat it instead.
It will get better with treatment.
The size of the lymph node is considered more than normal when it is felt to be more than a pea size. Then depending on the site one judges the severity. its not uncommon for lymph nodes to be inflamed. it can happen due to a variety of causes. any irritation or infection in an area too can cause lymph nodes in that region to swell, they are the filters, the sentries. but usually with the treatment , it goes back to normal. If it does not then it needs investigation.
I would recommend the intake of Tab. Tylenol once a day for 5 days, it will help to reduce the inflammation.
Also you can give warm compresses over it once a day with the help of a cloth dipped in warm water.
If it does not help within a couple of days, you will need an oral antibiotic like Cap. Augmentin. But that will require a prescription from your doctor. Hence I recommend seeing a doctor for the same.
Please let me know if you have any queries.
Hope my recommendations are helpful.