Hi there
I had EXACTLY what you are describing happen to me a few months ago - if you want to see the advise I got then go here:
http://www.babyexpert.com/chatroom/topic/29307It was basically a blocked 'sprayer' - the hole on your nipple the milk comes out of (no idea what it is called!) It took my little boy over 48 hours of ordinary feeding (he was 5 months and exclusively breastfed so feeding a good few times a day still) to unblock it and with a choke as whatever came out but that was that. SO other than keeping feeding I don't know what to suggest!