It its more the fact that so many kids i know can string together some sentences, or speak alot clearer and communicate what they want.
I used to stress about the exact same things. I know everyone says not to compare. But when it is your child that seems "behind" thier peers, it is near impossible not to.
Honestly, she sounds like she is doing beautifully, even if it is in her own time. It will come and she will catch up. Its ok to worry. Its our job, but try not to let it consume you, focus on what she can do, rather than what she cant. Take it from someone who wishes she could go back and see her clever little guy for who he was instead of who he should have been. I missed way to much because I was obsessing over this.

ETA- I hope that doesnt come across the wrong way, I totally get it and like I said, if you think there is something, get it checked out, there is no harm in that, worst (best) case scenario, you have a bit of piece of mind.

I couldnt remember, so I looked it up, everything I read said 2 word sentences at 2 years. DS did not say more than 10 "words" including animal sounds, at 18 months. Then suddenly at 20 months he just started copying, putting the odd couple of words together, etc. It was like something clicked.