my car did that after filling up and then letting it sit for about a month.. now my gauge is completely inaccurate...
i never got to fixing it so i cant tel you what exactly is going wrong but just know this is not a good sign

 

I was reading the posts thinking this was a new thread, I was about to say "GAS IS NOT CHEAP!". I only get about 5 gallons off 20 bucks nowadays.