Sometimes it can be a blocked sweat gland. I have had them a couple of times. Try to minimize or not use deodorant for a couple days on that side and it may go away on its own. If it is this and persists antibiotics can get rid of it.
Good luck to you !
Keep an eye on it. If it doesn't go away or causes pain, see a doctor. It could just be a swollen lymph node but it could also be a sign of something more serious.
Make note on a piece of paper that you spotted it today. Describe its location, what it looked like (if possible), what it felt like (hard, lumpy, fluid-filled, etc.), about how big it was, if it moves back and forth when you touch it or stays in place, if it hurts, etc. Then, check it. Any time you notice a change in size or that it's behaving differently in any way, make note on that same piece of paper with the date. After a month, go have it checked out if it's still there (especially if it's changed in size, etc.) and bring your paper.