It could be any number of things. Don't jump to assumptions until you've had a chance to speak with her. Just keep calling every thirty minutes or so and see if she picks up. If she doesn't and you've already left a message, hang up and try back later. When you do get to speak with her, all you have to say is "Sorry I had to work late and couldn't call you right away." Just ask where she's been. If she acts pissy at you just because you had to work late, maybe you should reconsider the relationship.
“This planet has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn't the small green pieces of paper that were unhappy.” ― Douglas Adams, The Hitchhiker's Guide to the Galaxy