I noticed a bump in armpit 2 days ago. Size 1cm x 1cm, felt tender. Today I got my period, and bump has shrunk, no tenderness. Is it hormones? See doc?

Lump in arm pit. Most of these kind of lumps in your arm pits represent blocked sweat glands, which can occur randomly in anyone. As long as they go away by themselves in 2-3 weeks, there is no special concern. These glands are sensitive to hormones (increased sweating), but the hormones are not really the cause. The cause is obstruction or blockage of the normal duct from the pore.