Ah, it was a disease then. Must've picked it up in Privateer's. Bleh. Regardless, this situation did teach me the importance of saving. So, not all's bad.
Actually, I think it's not a disease; it's poison. Some diseases will kill you quickly if you fast travel too far. Some work slower, and you have more time to travel. But no disease will kill you over the course of a rest before it so much as shows up in your status report.
Poison is another matter. Poison hurts you over a much shorter period of time, and there is no indication that it has occurred other than a quick "you have been poisoned" message that can be easily missed as you fight (IIRC)... that, and the periodic red flash followed by a sudden drop in hit points. Diseases erode your stats. Poisons act directly on your hit points, and can damage you faster than you can heal during a rest.