Piggies cen get pee stains but the fur should be dry... they shouldn't have a wet underneath (underneath for the boys, wet 'bottom' for the girls)- there's always a reason. Sometimes a urinary infection or other issue can lead to them dripping pee constantly or being unable to get a proper flow going. Sometimes it's just a bedding issue. I thought I had a lazy boy with his wet tummy (he was 3 1/2) and actually it turned out he did have a urine infection - easily treated with antibiotics and painkiller over 5 days. But we also found out at the vet that the reason he didn't move about much (and thus sat in his own doings, and thus got his infection) was that he had arthritis in his knees and he was a bit stiff. He stayed on the metacam painkiller and a food supplement that contained glucosamine (Oxbow joint support - I got it off Amazon) for the rest of his life. He lived till he was 6 and he was a lively little trooper until the end