I fill up my car with gas and it says low fuel?

i 03 buick rendezvous. i got the fuel pump fixed yesterday and then i went to get it cleaned. But before i got it cleaned i fulled it up all the way with gas. Now it says low fuel!! so i went to the gas station and but 10 dollars to see if i had a gas leak and then it said all the way full then BOOM like that it was low! Whats wrong pls help?