One tiny sore lymph node

Hi :) hoping someone has experienced this and can shed some light. I've had mastectomy, chemo , rays and some lymph nodes out. That would be coming up to 2 yrs ago. I don't have any trouble with my arm swelling etc. I exercise and lift fairly heavy weights. Now I'm not sure if it was there prior to my last workout but.... I worked on my chest area and noticed it was getting sore towards the end so I stopped. Last night I found one tiny but very tender lymph node on the outer part of my chest. Kinda above the breast that not there and out towards the from of my shoulder if hat makes sense. Anyway I've read that cancer is usually not sore, I spoke to the breast nurse and she didn't think the training would do it .... Any thoughts . Oh and it's hard very tiny and very sore. Xxxxxxxx kell