A verb-preposition combination becomes a "phrasal verb" when the combination creates a different meaning together than when those same words are used separately.  In your second example, "looked up" means "search" not to literally look up (wards,) as in toward the sky.  In the first example she literally looked back, as in behind her: therefore, it is not a phrasal verb.  I hope this helps.  Ask me if you have further questions.

Hi ioletta, you will need a proffesor of English language to answer that question. Native speakers are not worried about phrasal verbs. To ask this question suggests you have a good level english. Best of luck!