My armpits were itching like crazy about 2 months in - and they were bulbing out. I'm a high-risk case for several reasons, so I get sent to a specialist/surgeon for every weird thing that happens. For this, they sent me to a breast specialist only because I had been concerned that if this were being caused by toxins in the lymph system, I didn't want it affecting my breast milk.
I knew it wasn't my soap. I knew it wasn't my deodorant (both organic, hypo-allergenic, and natural).
TURNS OUT - mystery of the itchy armpits solved - your MAMMARY tissue actually goes up a ways into your armpits - about halfway. The mammary tissue up there is affected just like it is in your breasts. THAT'S why our armpits itch. I'm not sure why all women's armpits don't itch, as what the specialist showed and explained to me were how the mammary tissue extended up into the armpits. Maybe that was just an average of how the mammary tissue runs up there.
But I can tell you... given the information from an extremely dry, condescending, yet competent breast specialist... that the reason for the itching is mammary tissue that runs from the breast, under the arms, and about halfway into the pits being affected by the same hormones that are making your breasts swell and itch.