Is it during any specific time, like when you first latch lo, or if you haven't nursed in a while?
Is it in the breast tissue or nipple? Do you wear a bra with an underwire that may be causing some pressure on some of the milk ducts?
Let down can cause some pain, but from my experience it was more of a dull squeezing sensation. Thrush can cause sharp pain, I think after baby has nursed. Blocked ducts can be more like a feeling like someone punched you in the boob but it subsides usually after baby has emptied some milk.