I Am A Breast Cancer Patient (survivor). Had A Mastectomy

I am a breast cancer patient (survivor). Had a mastectomy on left breast May /17. Also chemo and radiation. Have been doing fine, but today noticed a red, warm to touch, swollen area on my left upper arm bicep. It is sore. I don't know what this is or whether it is something I need to worry about. It is Saturday night, and I don't want to go to the ER. What should I do?