If you aren't already doing it, I would strongly suggest putting a hot (as hot as is comfortable - don't burn yourself) compress ie washcloth or towel on your chest until it cools down and then pumping. This will help to open the ducts, won't affect the milk, and should give you more 'output'. The *bleep* actually do not empty all the way, even when you are feeding, but if you are still feeling a lot of pressure it sounds like you aren't expressing as much as you need to. You might have some blockage in one or more ducts, which is not uncommon, and the compresses will help with that also.
*If you notice redness, swelling, or start feeling feverish, call your doctor.*