K.P.
Lots of toddlers go through this stage around this same age. They are old enough to be aware of you leaving, but don't always believe that you will come back!
See if you can find someone she is comfortable with who is patient and willing to work with you- like an aunt or grandmother. Explain the situation to your helper and then make arrangements to run a short errand. When you leave the house, give her a hug and tell her that you will be back soon. NEVER sneak out b/c you are trying to work on building her trust that you will come back.
Leave the house for a short period of time- even if you just grab a cup of coffee and pick up some milk. When you get home, even if she's been sobbing for the entire 30 minutes, tell her you are so proud of her. Try to leave the house without her at least once a week and always with someone familiar. Eventually she will get through this phase!