Please do check if you reference the same controller extension in multiple pages, and you are trying to reference one page from another.

I had same issue today,  I was tried to send email with PDF attachment, I did created two pages, one for render PDF file, another for send email screen, those two pages are using same controller extension, from below send mail code, you will see, the controller extension have been refreshed because I want to render PDF as attachment, but the send mail screen still remain same - return null means does not redirect to any page, then i got the error message "This page has changed since you started your edit. Refresh your page and try again.".

the solution for me is split this into two controller extension, one for send mail screen, another for PDF attachment, then it is working fine.

hope this idea works for u.

	public pageReference send(){
		//Define the email
		Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
		
		
		ID QuotationId = ApexPages.currentPage().getParameters().get('id');		
		System.debug('Quote ID: ' + ApexPages.currentPage().getParameters().get('id'));
		
		
		
		//Reference the attachement page to pass in the quote ID
		PageReference pdf = new PageReference('/apex/QuotationPrintPDF?id='+ApexPages.currentPage().getParameters().get('id'));
		pdf.setRedirect(true);
		//take the pdf content
		Blob b = pdf.getContent();
		
		//Create the email attachment
		Messaging.EmailFileAttachment efa= new Messaging.EmailFileAttachment();
		efa.setFileName('xxxx.pdf');
		efa.setBody(b);
		
		List<String> toAddress = new List<String>();
		toAddress.add('qingsong.li@msn.com');
		email.setSubject('Email quote ');
		email.setToAddresses(toAddress);
		email.setHtmlBody('xxxx');
		email.setFileAttachments(new Messaging.Emailfileattachment[]{efa});
		
		//Sends the email
		Messaging.Sendemailresult[] r = Messaging.sendEmail(new Messaging.Singleemailmessage[] {email});
		return null;
	}