This Isn't What It Looks Like (The Secret Series #4) by Pseudonymous Bosch

From Little, Brown Books for Young Readers:

The Secret Series continues in this dangerous and daring fourth adventure.

Cass finds herself alone and disoriented, a stranger in a dream-like, medieval world. Where is she? Who is she? With the help of a long-lost relative, she begins to uncover clues and secrets–piecing together her family’s history as she fights her way back to the present world.

Meanwhile, back home, Cass is at the hospital in a deep coma. Max-Ernest knows she ate Time Travel Chocolate–and he’s determined to find a cure. Can our expert hypochondriac diagnose Cass’s condition before it’s too late? And will he have what it takes to save the survivalist?