I too unfortunately have a lot of recent experience. As Sagres said, a lot of the time they are harmless but in my experience it is better to be safe than sorry. The fact she is 3.5 with no previous issues is good, however, I would keep an eye on the fatigue. If it gets worse, please get it checked out earlier. Also watch her breathing, if she starts to struggle take her to the hospital. I'm sorry if i frighten you and please remember I am going from my own recent experience of a slight murmur indicating a much more serious issue. A murmur can be indicative of so many different things. It's hard to say from just that one symptom- basically just means they can hear a noise other than the heartbeat. Could be nothing, could be serious. Good on you for getting her checked out.