Lump under my arm?

UPDATE: I went to the doctor today and he told me it's a swollen lymph node. Which should go away within the month. He said it's probably because I recently was sick and my immune system was low. I'm not quite sure but u have a follow up appointment in a month.

The other day I felt a lump under my left armpit. It's not completely hard and it kinda rolls around. It's very sore. I do have a doctors appointment soon. And it feels about the size of a quarter. I'm just wondering what you ladies think. If it helps I'm 18 and mostly healthy. I think it's a swollen lymph node or strained muscle. Thank you