Turns out Diego-san had an abscess on his tushie and a slight fever. The vet said the abscess was likely caused by one of his brothers (not that I would ever name the prime suspect's name... Theo...) biting him. She gave him some anti-biotic and sent home some oral anti-biotic. $124 later we were back home, safe and sound. I may have to supervise Theo and Diego-san's 'together' time more closely. That might be quite the chore however given the size of our castle.