Hi Lubna, this can be confusing at first. “Mon, ma, mes, ton, ta, tes, son, sa, ses,” etc. are called possessive adjectives and adjectives modify nouns. Here, the noun is “le sac” which is masculine so it takes the masculine form of the possessive adjective which is “son.” It’s confusing because the subject pronoun is “elle” which is feminine. 

The subject pronoun (je, tu, il/elle/on, nous, vous, ils/elles) determines which form (can’t think of right word!) of the possessive adjective you use (like first person, third person singular etc.) but you’ve got that right. The subject pronoun “elle” uses “son, sa or ses.” “Je” would use “mon, ma or mes” and so on. But the noun determines whether you use the feminine, masculine or plural form of the possessive adjective.

Elle a son sac. = She has her bag. “son” because le sac is masculine 

But

Elle a sa voiture. = She has her car. “sa” because la voiture is feminine.

Hope this helps.