How old is the piggy?.If he is elderly,it could be incontinence.
As has already been suggested,trim the hair round the rear end so that it does not drag on the floor or into puddles or urine.There is no need to go to a vet you can do this yourself.
Some piggies do get messy round the rear anyway,I have 2 boars that do and they are not long haired.I wash the rear end every few days when necessary,there is no need to give a full bath.,but a bath in one of GGs shampoos every 5 or 6 weeks is a good idea anyway.
It could be a urninary infection,but there are normally other symptoms with this.