Hi, my cat has been having issues when he goes to the bathroom?

Hi, my cat has been having issues when he goes to the bathroom. There has been mucus-like fluid in his stool (sometimes excreting only the mucus) and going to the bathroom seems to be extremely painful for him (he winces and meows every time he goes). He was on one round of antibiotics, two rounds of Prednisone and is currently on Purina Fortiflora. He's also gotten blood tests, a fecal exam and x-rays and everything came out fine, yet nothing seems to be helping. Is there anything else we can do to help or do you know what might be causing this?