They don't have the same kind of memory we do so they don't forgive but also they don't hold grudges. They will remember unpleasant things so they can avoid that situation again, that's a survival mechanism.
If your piggy ahs had a bad experience just be patient and take time to show them that you are a safe person, talk gently to them, hand feed them treats, spend time just sitting by their cage talking to them. Take things slowly and they will remember to trust you.