Soft YTA

I don't think you really meant harm, but she's probably very embarrassed and your comments likely made that worse. Bodies are stupid and they don't act as expected sometimes. My cycles are usually pretty heavy, but they are consistent. I remember one time I was very sure my cycle was over so I went about my day, and BAM crime scene in my pants. Despite our best efforts, sometimes our bodies throw a curve ball at us.

I personally would have replaced the sheets anyway because I would not want a reminder of the embarrassment, and she likely had plans to buy a new set or wash them very well so you couldn't tell. If something like this happens again, comfort her and tell her it's okay. You can worry about the sheets later, and more than likely she will take care of it without you having to say anything just to get rid of her embarrassment l.