"hard upon you"

Hi!

What does "when someone is hard upon you" mean?
In the first place is there a phrase "hard upon"?

Thanks in advance.

Yes, there is. The Oxford English Dictionary defines "hard upon (on)" as "close before or after so as to press upon." Example: "It was now hard upon three o'clock" . For one person to be hard upon another in a literal sense would require some such setting as a footrace where two runners are close together on the track and the one behind is closing the gap. The phrase might apply figuratively to a business competitor, for instance, or to a detective hot on somebody's trail. I don't know whether the dictionary meaning fits the context where you found the phrase, though.