It's "em + as = nas" (preposition + article).
In Portuguese, the definite article is optional before possessives.
You can say "minhas roupas" as well as "as minhas roupas".
Since we have a preposition "em", it could be "em minhas roupas" or "nas minhas roupas". (Again, the article is optional).
Minha is the feminine version of mine/my. (Meu is masculine.) Minhas is the same as minha, but only used with plural nouns. Ex: Minha batata, minhas batatas. You're still saying "my" in both examples, but since the object changes (potato, potatoes) the word "my" does to. I hope this helps! :)