problems with notepad programming

sometimes when I write some code (html)in Notepad and save and close the file, the file
changes, somtimes it doesn’t. What I mean is that when I reopen the saved Notepad file
usually it is okay and I can continue writing and editing it, but once in a while the file
reopens and it has all kind of crazy scrambled code with capital letters mixed with lower
case and all the quotation marks missing, and all kinds of new code elements that I didn’t
put there. Usually when this happens I cannot straighten the page out and have to throw
it away and start all over rewriting the file from scratch.