Hi. I Have A Swollen Lymph Node In The Crease

Hi. I have a swollen lymph node in the crease of my left arm that has been getting bigger. Giving me some problems for about 3 months now. Causing some pain in arm, shoulder and into the shoulder blade.
A mammogram ruled out any concern for breast cancer, I just do not know what else could cause the lymph node to stay swollen.
I am not ill, nor have I had any kind of cold, sore throat or infection.
Just wondering if this is something that I should continue to ask my doctor about. Thank you for any information.