This is a sense of impending doom, which can be real or imagined.

Fear of impending doom anxiety symptom, from the Anxiety Centre site:

You feel as though something extremely bad is going to happen, but you are not sure what.

This is a sense of heightened anticipation for any stimulus expected anxiously, whether real or imagined.

Definition of anticipation, Collins English Dictionary, 12th Ed:]1

1. the act of anticipating; expectation, premonition, or foresight