Use your fingertips to feel for lumps. If you can detect any, its probably a blocked duct. Otherwise, then its probably just hard tissue. When I had that problem the last time, the LC did the cold cabbage treatment for me - but my case was really bad.
You are probably still experiencing hardness after pumping because there's still milk. When pumping, try hand expression to completely empty the breasts. I came across this video the other day that shows how: Maximizing Production - Newborn Nursery at LPCH - Stanford University School of Medicine
After a pumping session, your breasts should feel completely soft, no hardness anywhere. Also, it should be smaller in size.
Also, its perfectly normal for one side to produce more milk than the other. Depends on many factors like how much your baby has drank from it, etc.