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Thank you for your question. My name is***** I have been a small animal veterinarian for 13 years.
Unfortunately, nervousness is not going to cause a heart murmur. If there is a heart murmur present it can be made worse if the heart rate is faster. So, if a dog is nervous and therefore the heart is beating faster it can make a murmur sound louder, but there was likely a murmur there to start with.
I wish I had a better answer for you, but the vet that mentioned that she was very nervous was wrong to suggest that was the whole explanation for her murmur.