3 yr old with a heart murmur...what does this mean???

Hello, today I had my 3yr old son at a doctors appoint.(a allergy/asthma specialist)  After several minutes of listening to his heart beat the doctor told me he seems to have a heart murmur, and he would like to refer him to a cardiologist.   Our family doctor has never mentioned this.  From what I understand this can be common in young children but that it would most likely have been discovered before now.  I know  there is an 'okay' murmur but there are also those that can be problematic.  So my questions are, if it the kind that is not good what does that mean, what will be done????   Was he born with this or could he just all of a sudden gotten this?? What should I expect to be done at this appointment?  What questions should I be asking?   How concerned should I be???  
Please any advice or information you could provide me is greatly appreciated.
Thank you.  

[ireneo]

Find out what's causing the murmur (whether it's functional or from a valve for example) and how severe. I've had a murmur all my life. Even to my untrained ear I can hear it gurgle whoosh when I use a stethoscope. But I've had it all my life and it's no concern (I'm 57). 2 of my 3 grown kids have heart murmurs but again, they're mild and no great concern.

It's great that the doctor is going to follow up on this and make sure it's benign. But having a murmur doesn't necessarily mean something terrible is going on. That's the good news. I hope you get even more good news from the cardio.

[lifesconcerns]

Thank you for your response!  So this would be something he was born with than right?  Do you have to monitor it to see if gets worse or will it just always be the same? Or can it correctify (lol-not sure if that's really a word) itself?  I've been doing some reading on the internet but I find that just often gets me more nervous even though as you also mentioned alot of the time they are nothing to be concerned with...of course it's the ones that are of concern that worry me and I end up focusing on.
Yes this Dr. was amazing. My son was referred to him for allergy/asthma testing and he was just so thorough, and didn't seem to be any rush to push us out the door.  He somewhat restored my faith in doctors.  I am glad he went as far as sending out a referal to a cardiologist.  I will now 'wait' for that phone call for an appoint.   Always so much waiting.
Thanks again,
Take care and Happy Holidays