I was told that it is very common, esp around 2-3 years of age, and often just goes away by itself, BUT, don't be fobbed off by gp saying, oh it will just go, because it might, but might not, and will almost certainly benefit from eg speech therapy. If it's causing her such distress, maybe see gp sooner rather than later.
Try talking slower to her & looking full in face, so communication Is easier & less hurried & she doesn't feel under pressure, and of course, don't comment, tell her to take a deep breath & start again, complete sentences for her etc.
My ds has started stammering a couple of months ago (aged 2.5) he doesn't seem bothered tho, so I am biding my time, altho' have mentioned it to doc as was there already about sthing else
google stammer or stutter, lots of web site & orgs that can give you useful advice/tips/reassurance