When the bloat gets to the point of pain upon touch, there's usually no time to wait for any laxative to work.
As you've noticed, it's speeding along rather quickly - from being fine this morning to being in this condition right now.
If this is simple constipation, even at that it's many times a symptom of any one of many different health problems. It could be a perineal hernia, anal/rectal lesions, or stricture.
Obstipation is when fecal impaction makes defecation impossible. Megacolon is when the large intestine becomes distended. It's a frequently acquired disorder that results from obstruction by foreign objects, masses, injury or neurological deficits
Offering a soft canned food with 1/2 teaspoon pumpkin (not pie mix, the real 100% pumpkin), a sprinkle of bran cereal or some Metamucil, Konsil, Siblin (a few drops in their food) may provide a quicker solution.
There is a controversy about offering a couple drops of mineral oil. Some professionals strongly advise against it because it may be inhaled and cause severe problems; others feel it's just fine on occasion for episodes like this. I feel it's a personal choice. I've never had to use it, finding huge success with soft food diets in our cat rescue efforts.
By waiting, you may be facing surgery, so be prepared. This may be necessary to remove impacted fecal material and eventually to correct the cause of constipation (if that's the problem).
The two common surgeries are Colectomy or pelvic Osteotomy. The Colectomy is usually opted for because of the simplicity of it compared to the Pelvic Osteotomy with or without Colectomy.
Again, early intervention could provide a simple, quick and inexpensive solution. If you wait and this continues to worsen, well, it could end up requiring an emergency clinic and emergency surgery.