My DD had diarrhea last night. She woke and managed to get to the toilet but there was a fair amount on the sheets. I've washed the sheets and duvet cover. The mattress cover is so tight and I don't think it will go back on if I get it off to wash. Shoild I try? Also, there is a massive area of the mattress that was covered and I can see old stains too. Google says not to use bleach. What do I use and how? I have a steamer as well. It's a new mattress! Any suggestions welcome.