Why are we not allowed to brush our teeth or take a shower after having Holy Communion for at least 9 hours?
The reason as I understand it is to do all we can to protect the Holy Body and precious Blood we have received in Holy Communion. The idea being that if I brush my teeth and spit out the yukky water, I may also be spitting out some small fragments of my Lord's Body that may have remained lodged between my teeth. Similarly, if I have a shower and some soapy water goes in my mouth, I am likely to spit it out pretty quick.
This Q&A was found on Archangel Michael & St Bishoy Coptic Orthodox Church : http://www.stbishoy.org.au/modules/smartfaq/faq.php?faqid=59