Puppy's forgotten where to pee

[DudleyDud!]

My 4 1/2 month old puppy has been very reliable with peeing on his pee pads (we live in an apartment) since he was 3 months old. Just these few days he has been peeing on the carpet right in front of us and even when we direct him to his pad he moves away to pee somewhere else. He's currently on a course of antibiotics (Clavamox) as he just had a tooth extracted. I don't think it's UTI as he doesn't seem to be peeing more often than usual and his pee is clear and there's a lot of it at any one time! He's drinking water, usually drinks a lot anyway. Is it a case of the antibiotics working or is it a case of having to retrain him again?