SD cards have a finite number of times that they can be written and erased. Take it as a warning and get a new card before you lose more pictures. 
My big Panasonic camera started returning card errors this summer. Card was about five years old.
The Raspberry Pi computer that I have running and linking my weather station to the web uses a SD card as its hard drive, by default. Guys have found that the constant read/write imposed by the OS can lead to early failure of the cards. I am using a mechanical Hard drive in the Pi instead of the SD card and the computers have been rock solid. 
Newer computers that use solid state drives use a different type of memory and the drive uses a leveling algorithm to keep from "wearing out" a particular portion of the memory if it is the only part that is being written and erased. SD cards have no such provision, so they can be subject to early failure