sophyland,
I had this experience before and my doctor sugguested me to:
(1) Have a hot bath /shower
(2) use hot water bottle /wheat bag on the blocked ducts
(3) let the baby drink from the blocked side first (every feed). When the baby is hungry, the suction is stronger..
(4)massage the blocked side often (not just during the feeding time).
Hope these help...relax and be a happy breastfeeding mum....