Mon petit is definitely masculine. Ma petite is the feminine. Only if the noun that the possessive is modifying begins with a vowel does the possessive retain the masculine form as in mon ami and mon amie (although ma amie and m'amie is used in the vernacular sometimes, it's considered really crass, and L'Académie Française quivers in terror. *g*). Hee! Listen to me be all lecture-y. *embarrassed grin*

Forgetting all that, the nickname is cute. I'm glad that the two of you have retained that kind of freshness in your relationship.