You didn't mention if this was also the case for text or just numbers. (like if you wrote appl3, would only the 3 be backwards or all of them). Or if this only applied to existing numbers or also new numbers that you type.
So two possible answers. One, you are looking at the drawing from the bottom instead of the top. You viewcube will say bottom instead of the usual top - or the text in the upper left corner will say bottom.
Or you might have the MIRRTEXT variable set wrong at one time. Type in MIRRTEXT into the command line. Generally we like it to be set to 0 (zero) so it DOESN'T mirror text. But if you set it to 1, and mirror the backwards numbers, it should flip them the correct way. Then set MIRRTEXT back to 0 when you've fixed them all.
If neither of these helps, we need more info. At least a screenshot.