Try spending an afternoon with several short in/out trips. The goal is to establish a routine so your pup knows what to look for when you leave. If you have the leash, she gets to go. If not, she doesn't. You can't just say, "bye" and leave. You absolutely must make sure you have her attention so you can teach her the visual clues that she can rely on to tell her what is about to happen. No slipping out while she's napping. If she were to wake up and you were gone, she would be frightened.
Give the pup a scratch on the head and a kiss on her forehead and tell her you will be "right back", or "you have to stay home". Just be consistent with the phrase that tells her you are going and she is staying. Then leave, lock the door and walk around the building. Do it again and check the mailbox. Go again and drive around the corner. Go again and go to the end of the hall. You actually do have to leave the other side of the door. Your dog can smell and hear you, so if you are just waiting a few minutes to open the door again, well... you will have one very confused pup! Each time you return tell her hello (if she's greeting you at the door), scruff her head again, and then get on with putting everything down. If she's at the door crying and howling when you go in, look at her (don't scruff her head) and ask her, "what's wrong?".
Do this several times in one afternoon and your pup will begin to learn the visual clues that tell it you are leaving the house. She will begin to understand that sometimes you come right back and sometimes it takes a little longer. Mostly she just needs to know that you go in and out all the time and it's no big deal.