Pain deep in one breast

CDCbaby

LO has been BF'ing well since birth he is 2.5 weeks old I had the normal nipple pain in the beginning but that pretty much got better around the 2 week mark.

Well, for the past couple of days I have pains deep in my left breast whenever he nurses. It feels like a straining/pulling sensation, not really a "sharp" pain, if that makes sense. I pump occasionally after his morning feeding when I am still really full so I know that my left breast usually produces way more that the right.
And I don't have these pains when I pump.

I don't think it's thrush b/c there are no other signs of it. And I don't think it's clogged ducts.

Any other ideas/thoughts/suggestions on why I'm having this new pain?