dui arrest when not drunk?

i got arrested for dui today and i was not drunk. the officer asked me if i was drinking and i had 2 beers at lunch so i told him that. i got arrested 2 hours later. they made me use the breath machine and it did not register. i dont understand how they could arrest me because there is no way 2 beers 2 hours before could make me drunk and i told them that. The seargant said i was not going to be charged, but I had to pay to get my car out of impound. Do I need a lawyer? Should I have to pay for the impound? Will this go on my record? I have never even had a ticket.
i dont understand... i could have said i had 0 beers but that would have been lying. i think saying i had 2 beers was why i got arrested. i told them the time frame though.