why is "jai les cheveux longs et ondulés" not feminine since it's a girl talking?

It is not specifically the gender of the person that determines agreement but the grammatical gender of the noun - cheveux is invariably a masculine noun, so adjectives describing cheveux always take the masculine form. 

Body “parts” generally have invariable grammatical gender.

Bonjour,

If I'm not mistaken it all depends on the ending of the noun. 

Here use this link it will help

Identifying the gender of nouns by some specific word endings in French