Firstly try not to panic.
Pee will oxidise whenit comes into contact with the air, and various factors can affect how red it looks.
In the first instance you should start by checking the weight of all of your piggies.
Are all of their weights stable (assuming you weigh regularly and they are all adult pigs)?
Check their bottoms and genital area - are there any signs of blood or discoloration?
Are they eating normally and behaving ok?
If you have concerns about one pig in particular you can always place them in a washing basked on a pale coloured towel and watch closely - does their pee look red when it is fresh?
If you have any doubts then you need to take them to see a vet.
Blood in their urine is a symptom that needs ot be checked by a professional.