Cariña doesn't exist. It's not a word. If you call someone Love, Amor, it doesn't need gender. There are some words, like Sea, Mar, that have both genders, so you can say "Esta noche el mar está revuelto" and also "La mar está revuelta, esta noche". But Cariño, Amor, Mi Vida, Mi Cielo... they are nouns, substantives of the male gender that can be used on female receptor without having to change their gender. It happens with female words like Cosita, (little thing), or Vida mía, and I'm going to stop with the endearments because all this sweetnes is making me puke.

Summarizing, not all spanish nouns can be turned into female ones adding an -a. That only works with adjectives: Precioso, Preciosa; Muñeca, Muñeco; Guapa, Guapo...

I'm happy when I can help you. I've learned so much form you, it feels good when I can do something in return.