Very similar thing was picked up for our daughter when she was a similar age and was being seen by GP for throat infection. He assured us it was very common in young children when they had a temperature.
We were advised to go back a few weeks later, once she was fully over the throat infection, to have it checked again. She still had the heart murmur then, so we totally panicked of course.
Our GP was hugely reassuring about how common this is in young children and is almost always fine, but we were referred for checks at the hospital. Whilst we waited for our appointment, we sought the advice of a cardiologist friend. He again told us that it is so common for children to have it, then it resolves by itself, that it is almost not considered an issue (he phrased it better than that).
Sure enough, by the time we had the hospital appointment, it was fully resolved - they couldn't detect any murmur at all. She's had absolutely no problem since. (She's now seven.)
By the way, have you tried putting your ear to your child's chest and listening yourself? Although I am not medically qualified, I could actually hear the issue quite distinctly when our daughter had the problem. I've occasionally checked since then, and never heard it again. Obviously, I'm not for a second suggesting that this should ever replace checks by medical people, but thought you might find it of interest anyway.
I sincerely hope things work out well and that you can put this behind you.