I'm slightly confused with the history so could you first please clarify:
Both had a cold and Piglet was treated with antibiotics. When was this, what antibiotic was given, what dose and for how long for? What date was the last dose given?
You have now noticed blood in the cage and have taken them to the vets again and they have both been prescribed ceporex.
As already pointed out , ceporex is not the first choice for a guinea pig as it can lead to enterotoxaemia. However it is still quoted in the most recent Veterinary Formulary at an oral dose of 25mg/kg for guinea pigs so the vet is not doing anything wrong in prescribing it (just not very cavy savvy)
Having said that, most cavy savvy vets would not give this antibioitic except as a last resort if the bug causing the infection had been cultured, identified and was found to be resistant to the other antibiotics that should be used first (namely baytril, septrin, azithromycin). Even in this case, my cavy savvy vet stated he would only give ceporex by injection and not orally because of the risks.
There has been a very recent case I have been personally involved in where this drug was precribed for several piggies who all developed a bad reaction to it involving lethargy, not eating, blood in the urine and one had diarrhoea. Luckily they were only on the drug for couple of days but nevertheless had to be admitted for intestive care by another more cavy-savvy vet for three days before they were back to normal
Unless Piglet was given ceporex (or other harmful antibioitic) previously for his cold, it is unlikely that the current blood in the urine is a reaction ot the ceporex and more likely to be a UTI or possible bladder sludge/stone. However it is important that you stop this drug immediately and either go back to your vet and take the dangerous anitbiotics list with you or find another vet to prescribe septrin as this is considered better for dealing with UTI's than baytril.
HTH
x
Edit - you;ve just clarified Piglet was on the same drug last week - stop the drug now., in all porbaility it is the drug that has caused the blood in the urine. Can you put him on a white towel and monitor him for an hour or so to confirm he is the one suffering? Jenny at Tilehurst vets is cavy savvy
http://www.activevetcare.co.uk/practices/tilehurst.asp