Clavamox Help UTI

My cat woke up on monday and was trying to urinate but nothing would come out, so i went to the vet the same day and he did a few tests and sent out some urine for analysis. The results I have not got back yet. He told me it could be stones, UTI, Diabetes related, or worse kidneys starting to fail.
He gave him an anti-inflammatory and anti-biotic shot and squeezed urine out of him. He sent me home with Clavamox 125mg pills... He said give him one pill twice a day?? My cat only weighs 12pounds... is this an overdose?
He is peeing on his own but next to the litter box and not in it. He doesnt seem to be his self, he will walk around for a minute then lay down and will lick his mouth and paws alot. But early in the day he purrs and even ran around. Any ideas? is this clavamox dose too much?