A heart murmur itself in a kitten isn't diagnostic of anything - you would need more testing to know for sure what it is....it could be an innocent murmur (that means there is a murmur but it has no effect on the cat) or it could be heart disease (ex: cardiomyopathy). A lot of times, kittens will be born with murmurs and they will disappear by about 6-8 months. Some cats live with murmurs their whole lives and have no problems. (sometimes, the murmur is just a fluke - and you won't hear it anymore)
To test for heart disease, you'd need a chest x-ray to see if the heart is enlarged (which would indicate congestive heart failure), and also a heart ultrasound (at a kitty cardiologist) to see how the heart is functioning (sizes of ventricles and stuff). If there's heart disease, there are some treatments (which I am not familiar with enough to detail). If there is not, there is nothing you have to do.
My kitten has a grade 1 murmur and I am choosing to just monitor it for change and to see if it goes away by 8 months. I am also monitoring her for symptoms - looking for her having trouble breathing, changes in eating habits, less playing/other reductions in activity level, etc. So far she is one of the most active kittens I've ever had and eats like a horse - she seems really healthy.
The grade of the murmur doesn't really indicate the severity. Any grade could potentially be heart disease. However, it is less common that a high grade murmur is innocent. (your kitty's murmur could have just been louder, though, because he/she was nervous)
I hope the murmur is just an innocent one!