lmf brilliant! DDs and I enjoyed them, for very different reasons!
hawthers can I give you a regime?! So, rest as much as humanly possible (I know, I know). Then feed from the affected side first, using wet heat while you feed. Massaging your breast while feeding can help, as will feeding every time he squeaks! Babies are usually best at clearing blocked ducts, but you can also try hand expressing between feeds too, again with wet heat, in the shower can be good! Take paracetamol/ibuprofen for the pain if you want to, both ar fine while bfing. Watch out for mastitis signs (fever, breast soreness and redness, feeling shivery/shakey and extra tired) and get to the GP at the first sign.
When you have cleared it, it is worth seeing if you can figure out any causes. Sometimes it isn't anything obvious, but bras and slings can cause blocked ducts if they are pressing in anywhere.
I am having a shitty day. Had to bring DDs (who are close to being renamed something uncomplimentary) home straight after lunch as their behaviour was bloody horrible. DD2 was growling at her friend, DD1 refused to eat any lunch as she wanted ice cream. We had agreed to ice cream after they had finished playing, I warned her that she would be hungry if she refused to eat...so she was howling about being hungry. Miri was angelic of course...but (in typical newborn sod's law fashion) was hugely sick the very second I put her in the car. Joy. this set the tone for the afternoon, the big ones have been horrible to each other and me all day 
I am now refusing to allow them to paint, because I am too buggering tired to clear up after them. I will be flinging omelettes in their general direction in a minute, then hiding in the kitchen to eat chocolate. And goes the diet!