Why do I get other people's Facebook notifications on Instagram?

[Lovely101]

I keep getting a message saying you're "Facebook" friend has join Instagram and it tells you their username I don't even know that person or have them as friends but my old friend does have them as her friends!!! And then I think she gets into my Instagram too and changes my email and comments on my pictures like if I was the one commenting on them!!! Why does it do that??? Help plz

[Gracie]

There is definitely something wrong....either delete the account or talk to your friend.