When filling up my gas tank, something leaks from the bottom of my car? What's going on?

I have a 2005 Nissan Sentra. The gas
cap is a little faulty. The engine light
will come on if I don't turn it enough.
I was pumping gas when a man said
"it looks like the gas is going straight
out of the bottom of your car." And it
did, but it seemed to fill up fight, it's
not leaking gas any other time, just
when I'm pumping gas. Is it a
pressure problem? Do I need to
replace the cap? Thanks!