[ice_lazer3]

Unless the nickname is somehow suggestive, I don't think there's a legitimate reason for you to stop using the nickname. But yeah, make sure your gf understands that a nickname does not equal liking that person and that things between you and your ex are completely platonic. Also keep in mind jealousy's a fairly common thing, and depending on how long you and your current gf have been going out for, seeing how close you and your ex may cause her to be a bit mad. (I don't think that's a reason to stop hanging out with your ex, this just means you have to talk this through with your gf.)

Maybe you can even try giving your current gf a nickname :P