I don't know the girl, so I can't speak for her reasons. Every girl is different. I can say that some people are more empathetic and romantic than others. How long has it been since the breakup? If it's more than a year and she's still sending happy birthdays, maybe there's more there than just being nice. But it depends on why the break up and who this girl is, and if she's moved on with another love or if she's alone. It's not a cut and dry answer.