Help me understand how someone was able to "follow me" on instagram...

I have never shared my instagram with anyone.
I don't even use it, just to look at things I'm interested in.

I have a cousin who is on my facebook and is follwing me on instagram...

I have not advertised what my instagram is.

Where is the correlation???

I hate internet
Feels like an invasion of privacy.

I wanted the two to be separate