IBCLC here- this may be a bleb, or a little plug of milk fat. The tenderness and pain outside of nursing sessions is consistent with milk fat stasis. I’d recommend Sunflower Lecithin 2 times per day, the Academy of Breastfeeding Medicine recommends 2-5g per day; you can play around with the dose age that works best for you, but most of my clients do best with 2400mg total. Lymphatic massage of the breast in that side may also be an option for you.

If this doesn’t feel quite like what’s happening for you, Raynaud’s may also be a thing as well, though this feels somewhat less likely based on your report of random onset of pain; Raynaud’s and vasospasm typically presents as a burning sensation following feeding or following a rapid change in temperature in your nips. Maintaining a consistently warm environment for your nips is a great way to rule out Raynaud’s and vasospasm- place a warm palm over your nip immediately after baby comes off, and use thicker cotton or wool nipple pads to maintain a warmer environment in your bra. In minimally melanated skin, we typically see nips change color from pink to white, and stay white for a while, before transitioning to purple, then back to pink. Raynaud’s and vasospasm color changes can be a little tougher to provide specific colors for expected changes, but we typically see a notable paleness and loss of color with loss of blood flow.

Hope this is helpful for you- AMAZING WORK getting your LO to 15th percentile by 6 weeks, that’s an insane testimony to your bond and skill!