Is there any other natural expression in this situation?
This was my conversation with my friend today:
"Hey, you study so hard nowadays.
Whenever I went to the library, I saw you in the same desk"
"No....it was that I was just coincidently there when you came to there,
I studied only 1 hour per a day"
You could say any of the following:
"It was probably a coincidence, since I was only there for an hour each day."
"It was probably by chance, since I was only there for an hour."
*I don't often hear people use coincidental/coincidently in everyday conversation. I hear 'by chance' quite often in the US, but I'm not sure about New Zealand!
If you want to be funny you could say:
"Are you trying to hide the fact that you are secretly stalking me? "
"Are you following me?"