It could be that she's thinking about you. We're all connected and when we've shared a close bond with someone we can still pick up on their energy even when apart. Could it also be that you're used to sharing your happiness with her so when something positive or special happens you're reminded that she's not there to share it with?

Sometimes that happens to me too. I don't get a sudden feeling of sadness, but I do get a feeling that my ex is thinking about me. I can't really explain it, I just know it. I wouldn't question it too much, just stay on your path and keep focusing on what you want.