Lump in arm

Good morning.

I had a bilateral mastectomy in November after being diagnosed with IDC in my left breast. I had a 6 cm tumor and had 9 positive lymph nodes out of the 15 they removed.

This morning I discovered a painful lump about the size of a pea under the skin of the inside of my left arm, near where my arm bends and about 6-8 inches down from my armpit. The spot is red. My arm from my elbow to my wrist is swollen

I am now terrified the cancer has spread there.

I started AC chemo on Dec. 14. I'll have my third treatment on Friday.

Has anyone else experienced this?

Thanks for the help.

Scared and confused as always.