E-man, as a cat lover, and having cats for many years, he may have a health problem.
Is he drinking lots and lots of water too? If so he may have either diabetes or kidney problems.
He may also have FUS Feline Urinary Syndrome - urinary crystals that block up his ureture.
Take him to the vet. Quick! All three are deadly.
If you're really lucky, he's just having fun in the mud. But I doubt it.
Maxmillian SanQueue (17 lb Manx - buff and white) had the same problem 2-1/2 years ago. He was 7-1/2, and a healthy 17 pounds. I watched it for a couple weeks. He always was able to pee, but copious amounts along with drinking much water. He's diabetic! I now give him 2 insulin shots a day.
Hugs and Purrs
Brenda, Kevan, Max, Dominic, Princess