I have an old house (1858) that was wired in the 30's with romex.

Describe what you're calling 'romex'. It's very unlikely what you have
there is the same thing considered 'romex' today.

I was
changing a light fixture. I turned the power off to the circuit removed
the old fixture and while I was wiring in the new fixture I received a
slight shock when I touch the metal box and one of the wires.
Any ideas?

All sorts of ideas. Most likely a miswiring of the circuit such that it's
shared with another breaker. Half the leg going to one breaker and the
other on another. Usually happens when someone tries tying in another
switch or outlet inside an already crowded wall box (or they're just plain
stupid).

Test the other breakers to see which one is set up wrong. It may lead to an
obvious solution that can be easily fixed.

-Bill Kearney