What is the device you have connected to the 5v rail? That way we can look up the datasheet and see how much current it is expected to draw. Could the connected device have failed? It's not uncommon for burned out devices to act like a near short, drawing much more current than they do when working correctly.
Also, you can get more current out of the 5v rail when powered off barrel jack than USB - USB is limited to 500mA typically, but you can get around twice that out of the regulator, for a short time (limited by heat dissipation).