Talk to your breast care nurse if you have one. If you think you have an infection get to your GP, or any GP or medical service, in the morning. As a cancer patient you will get preferential bookings which is one of the few up sides of BC. Not worth the trauma, but there you go.
Armpits are awkward things. There are critical nerve junctions in there and they are also a repository for all manner of fluids after any insult to the surrounding tissue. Take the pain meds if you have any to see you through the night. Get advice. Mxx