I think you are probably right when you said it was about over-stimulisation. Can you perhaps put her in her pram and put a rug over it to block out the world when you are out? Maybe try to put her in her pram like this for one nap a day at home so she gets used to it, then it will seem safe when she is out.

It probably will get better with time. The world is pretty overwhelming for a baby, they really don't have much of a way to make sense of it, so it can just seem scary if they are at all sensitive. As she gets older, she will be able to cope with more.