3 year old had a heart murmur for the first time at her check up. i cant help but worry it could be something serious. she was running around in the office before her appointment. could this have caused it?

Most murmur "bad" will be picked up on day one or rarely within the first 4-6 mo. Over the years I found that if I listen to any normal kid long enough, I will find an extra sound in most. Most are evidence of blood flow, but some are just the dynamic action of the heart within the heart sac. If doc is curious s/he may get an ultrasound to rule out a problem.In most, this is not necessary.

Discuss the type of murmur with your Pediatrician. Watch for signs of respiratory difficulty, poor weight gain and sweating with feeds. Most often, I agree that the murmur is benign and requires no further evaluation.