I personally have never heard that. I know one person who was very, very sick at time of her ET and she got a BFP. By Flu symptoms - if you mean headache, nausea, etc. that could all mean you are preg.
There is really no way to tell until test day.
Lisa B
Age: 35
Blocked Tubes
First IVF cycle Mar/Apr 06 - BFP!! DD born December 2006.
Left tube removed 2007
Ectopic Pregancy naturally in June 2008
Right tube removed 2009
Round 2 IVF - Oct/Nov 2010 - BFP!!!