Is there anyone that your child was around in the family who stutters?
As Annie said, try your best to not allow your child to notice it, the more you stress it the more it will effect her and more than likely it will then get worse. The less you think about it, it may just go away. If not, it is okay, as she can take speech classes.
I used to stutter really bad as a child, but that was due to my father, as he stuttered extremely bad because of a heart attack and stroke. The more conversations a person has, the better their speech gets. I only stutter if I get very excited about something...
I don't know why at 2 1/2, but when my son began elongating his vowels at about 18 months, I read up on it. The main thing all the experts said was not to treat it as though you notice. In other words, if a child does a word irregularly (either by drawing out the vowel or by repeating the first or last syllable, or whatever), one is just supposed to react as though she did the word right, and continue the conversation normally. This reduces the kid's stress and somehow does not nail the issue into place so it becomes a continuing problem. It worried me when my son went through it, but it did pass. Google "stuttering" and you should find a world of good information.