Kitty - Poor you - I get this about every 6 weeks, usually from a crap latch.
I do the following, which always eeems to work:
Massage lump very hard before each feed - literally getting the lump between two fingers/nails and breaking up the lump of blocked milk. Use a lot of pressure to force the lumps to break up, even if it hurts a bit
Hot shower after each feed - put hot/cold/hot onto lumpy spot and massage firmly towards nip, to push out any milk left after the feed (don't worry if it doesn't drip out, you are still freeing it up from the ducts)
Loads of feeding on that side - once an hour if neccessary - (the best medicine) - massaging and combing a la Mossy each time
have you got any breast nurses? those plastic semi circles full of gel, to heat up and place on the lumpy bit? Do this (or use flannels) before each feed and the heat will loosen up the milk
You may notice some creamy bits round Ronnie's mouth after feeds - which are the lumpy bits of milk so that's a good sign, as it means he is drawing them out
When it happens to me I just stay in all day or even for 2 days and just feed/massage/feed
The hot/cold shower and a dose of Ibuprofen should reduce the swelling you get around the lump, which will help you identify the offeding spot. Mine always get very distended when they are blocked
Good luck
[hugs]
xxx