If I have been previously diagnosed with depression, should I be concerned about post partum depression?

Yes. Women with depression before pregnancy are more likely to get postpartum depression, even if it has been years since depression. It is a good idea to let your doctor know about your previous depression. You may also want to start working with a therapist now to help cope with all the changes during pregnancy and shortly afterwards.
There is Risk. There is evidence that women who have had past episodes of depression are at a higher risk of developing postpartum depression. This does not mean that you are certain to have it, but it means you need to be aware of the possibility.