Why didn't Tilly go to Cass when she saw her in pain in the moonlight?

Tilly did not go to Cass because she realized that she was having a vision.

In the deepest part of the night, I came awake to see the trundle empty and Cass over on the windowsill. Her face was silvery wet with moonlight. She stared at what wasn't there, and I saw in her slump that it was the old Cass come back, Cass having a fearful vision.