Had to refresh my memory, too...from Wikipedia:
Pam was severely injured in a car accident in the 1986–1987 season finale, and left Bobby and Christopher due to her apparent inability to let them see her in such a physically disfigured fashion. Nevertheless, while Victoria Principal never returned again to the series as Pam during its final four years before cancellation, Margaret Michaels, a Principal look-alike, played the character in the season premiere of 1988–1989. Then it's revealed that Pam is dying of a disease, but nobody knows about it except from her doctor. Pam is never seen back in Dallas again.
I remembered the car accident, but not the fact that someone else played Pam for a season.