What is the difference between notre and nos? Is one feminin and the other masculine
"chemise" is feminine, the possessive adjective must agree with the thing owned, not with the owner.
singular: "notre chemise" ; plural "nos chemises"
Note: notre/nos is the same for masculine or feminine nouns
why nos & not nous?
You need a possessive adjective here: our shirts = nos chemises
"nous" is a personal pronoun: nous avons des chemises = we have shirts