Don't know if this helps but we all have lymph glands under our arms....when I have a cold coming on or my body is going through a stressful time I get tender lumps under boths arms. They are both very visible and tender to touch but the right arm is always the worst, and I am right handed. They are very clearly under my arms and not on my breasts.

I dose up on my vitamins and rest a LOT and they go down.

Maybe something similar? I know it has nothing to do with breastfeeding but if you're a bit stressed out at the moment it might explain it.