I see you have gone offline and I will be going offline soon so I wanted to leave you an answer in case you return while I am away.
As you may know instagram is a photo sharing app. By default photos are viewable by anyone.
Because of this I strongly suggest you make sure her account is set as private. Meaning only approved people may view the picture.
To do this, Go to your profile by tapping the icon in the lower-right corner that looks like the image below:
Tap Edit Your Profile next to your profile picture
On iPhone/iPad: Scroll down to Photos Are Private and toggle the switch to On
If using Android: Check the box next to Photos are Private.
Telling her about the dangers online is a great idea. Like anything sometimes it can be difficult for kids to really comprehend how dangerous their online activity could be.
I like to always let them know that if they post a picture that they wouldn't want people to see on the internet it is very possible that the picture could get downloaded and spread all over the internet. When this happens it is impossible to get removed.
So just discussing that would be a good idea.
Also there are a lot of dangerous people who pray on kids. A good rule of thumb for a 14 year old is if they don't know them in real life do not share any information with them. Ideally though, they should share no personal information with anyone online
It is incredibly easy for someone to make a fake profile online so there is always a chance you are not speaking to who you think you are.
Just so you know, you can view what she is posting on instagram on this website:http://statigr.am/
Just search her instagram user name in the top box and it will pull up all of the photos. If she makes her photos private then they would not show up here. But part of keeping her safe is knowing what she is posting.
You may want to read through this article as well for some more information on keeping kids safe online in general:http://www.scholastic.com/parents/resources/article/your-child-technology/keeping-kids-safe-online
If you need further help please do not hesitate to reply back and let me know.