Separation anxiety at this age can be a sign that your daughter is reaching a new, cognitive milestone that enables her to have a more involved thought process: "What is mom doing when she's not with me? Does she miss me?" And here's the scariest thought of all that she likely doesn't have the language to express: "What if she doesn't come back? Who will take care of me?" The other cause of the behaviors you're describing could be changes in the family, especially parental stress. Are you or your finance stressed about something? Finances? Lost job/new job/stress at work? Almost anything that stresses us out, combined with a new-found (albeit age-limited) ability to reason, could be the cause of her anxiety when you leave her. Of course, when this happens, it only makes you even more stressed out!
Here are some ways to help her:
1. As much as possible, create and maintain a routine that is predictable, and talk about it. When my son was this age and for many years, my husband and I would create a narrative at bedtime of his day. We would list everything that happened: "This morning, when you woke up, you played in your crib and then you called for us, and then mommy got you and we changed your diaper and had breakfast...." Some days, when I missed a detail, he would remind me about it. Among other things, the narrative gives a predictability to life. It takes time to see the results of doing this but when she sees there is a sequence to the day and that it happens day after day she will begin to develop a sense of trust.
2. Help her develop coping skills. (1) "Here's a picture of our family. When you look at it, you can think about something fun we will do together." (2) Give her something of yours that she associates with you and might have your scent on it: "Here's mommy's favorite xxx. You can keep it with you." (3) Remind her of her coping skills: "When daddy is gone, teddy bear will be with you. He will keep you company."
3. Verbalize and validate her worries. The simple act of restating her feelings -- "You don't want me to go. You wish I could stay home." -- lets her know she has been understood. That alone can make her calmer.
4. Lots of the adult thoughts we express are not helpful. Don't lose your patience by saying, "I come home every day, don't I?!" and don't dismiss her anxiety by saying, "You're being silly. Stop it!" None of it computes.
5. Remind her of what will happen while you are gone: "You'll be with XX. After lunch, you'll take a nap with teddy and when you wake up, you'll play with XX. And then, guess what? Mommy will come home, just like always."