T.D.
To you and to everyone with the same problem, I would ask how the baby was latching on the breast. If they were for some reason not latching as well as they did on the other one, this can create the decreased size (not getting the milk) and the baby's prefrence for the opposite breast.
Make sure the baby's chin is touching the breast, but not the nose. Most of the areola should be in the baby's mouth. Start on the small side to recover, then feed on the larger size. Do start the feed occasionally on the larger size so the baby gets both fore and hind milk. They have different fat, protein and water contents. I would probably do two feeds on the small side the one on the larger side.
Please do keep nursing it is a wonderful thing for you and your baby. I breastfed my first for 27 months and my 2nd baby is two months old. I am also an aspiring lactation consutant, and love every minute. Hope this helps, if you have anymore qeustions you can contact La Leche Legue in your area.
T.