My sweet Mabeline Fabulash ("Mable") had heart issues. Her first vet "thought she heard" a heart murmur - but the woman was such a nightmare, I completely discarded her comments. I am grateful that I did. I learned five years later that I wouldn't have been able to do a thing about it anyway. Not a thing. Mable's heart issue was a congenital failure and would have been inoperable even if I could find a surgeon to do the work. (I think there's one in Oregon...?) There's medication if bigger problems occur, of course and Mable was on those for a time. She was THE BEST DOG in the universe.
And yes, if it's slight as your vet has suggested, she'll grow out of it! Good food, sunshine and exercise - plus all your good energy.... it's nothing to worry about now.
My suggestion? Just watch it (every time she has a check up, be sure the vet listens closely) and otherwise forget about it and enjoy your baby. Don't feel sad about it. Don't worry about it. It will rob you of today's joy.