I've never heard of this as a pregnancy related thing maybe its something you are using that's causing irritation and sensitivity as with pregnancy your everything changes including your skin so it could be your deodorant maybe switch to a sensitive one x
Just had refer back to my human anatomy days you have lymph nodes under your armpits also glands so because your breast are so close to your armpits and apart of the lymphatic system it can cause soreness. With my first pregnancy when my breast started changing it was painful in fact that entire area was sore almost like breast engorgement which makes everything sore including underarm glands I found heat pads were helpful.
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