A possibility is that the image counter has rolled over and the camera is re-using the old numbers (though normally camera start a new directory when this happens), or somehow due to battery fault the camera has restarted it numbering.

Or the card is faking its real capacity (frequent in no-name cards, and knock-offs), and now that you have written all the real parts, it erases old stuff to make space for your new images.

In any case you cannot trust a SD card for long term storage, especially if is often written to. You should export your pictures to some device for archival (typically to PC first, for culling, and then archive/backup), and empty the card before reusing it.

And yes, batteries wear out (especially if you let them discharge due to infrequent use). Batteries and SD cards are consumables. For SD cards you should stick to brand names (Sandisk, Kingston...) from established vendors (Amazon itself is OK, Amazon marketplace, much less so), for batteries there are excellent aftermarket ones what are less expensive than the camera company ones.