If it keeps happening, then you've probably messed around with the .dat-files, otherwise this shouldn't be so frequently. Or your save has become corrupted. That basically means that something damaged the file, removing entire chunks from the memory, thus causing Minecraft to re-generate them when you enter them. I have had such a corruption once, when my laptop crashed. I lost everything, but managed to get back my Minecraft save-files, and encountered the same problem. Whole chunks were gone and I lost a huge part of a map. I recommend making back-ups regularly and re-installing Minecraft every so weeks.