Passage from a book

I'm translating this short story and am struggling with phrasing the ending -

Entonces, oiste un largo suspiro. Aldo parecido a una inspiración profunda, indescifrable. No supiste si de cansancio o terror. Pero eras tú.

Particularly the last sentence.

Then you heard a long sigh. Something like a deep inhalation, indecipherable. You could not even tell if you were getting either tired or terrified. But it was you!