I have no idea where that idea has come from unless it is just a local thing. I can't really understand why tomboys would use it any more than anyone else. Girls who behave more like boys (that's what tomboys are) usually go down the route of not expressing their feelings, in order to seem more boy-like. Unless it's a very specific implied meaning locally to you, then I think the person who told you this gave you incorrect information. I can't see any logic behind a statement which means "I haven't seen you for a long time and that has made me a little bit sad" could possibly mean "I am very beautiful".
