Confusion in "-ing" form verbs use

I am a reading Jane Eyre book published by Oxford Bookworms.

I have found this sentence:

[...], while I lay awake all night, trembling with fear, and eyes wide open in horror, imagining ghosts in every corner.

I do not know when to use "-ing" like trembling and imagining. There is no be verb before them and I cannot distinguish any relative clauses (I know be verbs can be omitted in some cases when there is relative clause.).

Would you please guide me a bit in this case?

Thank you in advance