Any chance she has seen something or overheard something that scares her? Maybe seen something on the news about a missing or kidnapped child (Haleigh Cummings story)? She may not think consciously about it, or remember the specifics, but it could trigger her to be afraid to be separated from you and your husband.
Any loss in the family? (family pet, relative, neighbor) or loss by one of her classmates that might have talked to her about losing someone?
Of course, there is always the concern that something has happened at school. Have you asked her teachers about any changes at the school, particularly anything that might have changed or happened about 3 days ago? Or spoken to any of the other parents from your daughter's class?
Maybe you could let her take a photo of you and your husband with her in her backpack or snack bag when she goes to school. Or a "lovey" that she could keep in her backpack that she could take out to hold if she is upset. Just knowing her "friend" is with her might help.
I hope you are able to figure it out soon, so she can get back to enjoying school and you can get back to enjoying her instead of being stressed out.
Perhaps you could monitor her discreetly at school one day? Don't announce it to the school, either. Just drop her off.. drive off, then come back and go to the office, telling them you want to "secretly" watch her today without her knowledge... So you can try to figure out what has caused her to change her behavior at drop off. If it becomes an issue, as that the teacher also not know you are watching... or she might unwittingly change her behavior as well... - not that you are trying to "catch" her at anything, but if she is not behaving normally/naturally, you will not see how she interacts with your daughter daily...