blocked ducts in your nipples.
you can try put a warm towel on your nipple/breast.
then press down onto your ribcage n rub in circular motion (rub the lump).
then you use your finger n thumb to 'pinch' your nipple, pulling it outwards (like milking cow).
it should help release the milk that's stuck there.
alternate the above with pumping and it'll help to clear up the blocked duct.
btw, before you start the above, take a clean moist cotton wool to clean your nipples, rub away the dead skin that might be clogging the milk ducts.