Please have your piggies seen by a vet. It is most likely a urine infection in the wake of all this up and down weather that we've been having lately. Your vet will test the urine for traces of blood, so you may want to take all piggies you suspect could be the the culprit if you cannot find our who it is if they refuse to make bloody pees when you separate them and have them on a light towel or fleece.
Cystitis (inflammation of the bladder walls) or bladder stones/sludge or other possible causes that present with very similar symptoms.With sows, it can also be a womb problem.
If it is UTI (urinary tract infection), please ask your vet for metacam (painkiller and anti-inflammatory). Septrin the most effective antibiotic with urinary issues; it has also the advanatage that it is a lot better tasting and therefore easier to give.
If your diagnosis is UTI, you can help to get past the acute symptoms more quickly by feeding more mildly diuretic veg like fresh dandelion, cucumber, lettuce and chopped celery, but not so much as to cause diarrhea, please! This helps additionally to flush out the bladder. The metacam will also help with the discomfort. if there is any doubt, please ask for an x-ray to exclude bladder stones or sludge.