Amines from protein digestion. Eat like kitty, pee like kitty.
Is your cat's box in your bathroom?
The fact that you had just binged on carbs on a previously VLC diet suggests that the smell is of bacterial origin. I'd be willing to bet it is only a short term effect of a rapidly altered gut flora.
If you feel the need to binge, you should realize that VLC is probably not comfortably sustainable in the long-term. I would strongly recommend incorporating some sugars into your diet.
Apparently the thing in cat urine that makes it smell of cat urine is 3-mercapto-3-methylbutan-1-ol (MMB). You must have either eaten a food that breaks down to make that, or bacteria in your gut made it. I'm not sure what its precursors could be or how it can be synthesized.
Edit: It looks like it's found in raw onions, so maybe you ate a lot of undercooked onions that day... Or, you ate onions along with a lot of food and didn't digest the onions well enough, so the MMB didn't get properly broken down.
I do not know if our urine is made of the chemicals as Cat urine which consists of mainly three parts. Urea makes the urine sticky and allows the other two parts to stick around. Urochrome gives the urine its ugly yellow color that stains carpets, furniture, the wall and anything the cat sprays or urinates on. The third main part of the urine is Uric Acid. Uric acid in the urine consists of salts and crystals that give off the extremely pungent odor creating a gaseous state that attaches to positive ions floating in the air. Each of these ions has a positive electrical charge because it is missing an electron.
Starting 5 months ago my urine has started having an odor to it, and also is a little darker than before. What's interesting is that I'm able to tell what I ate previously by the odor of my urine, i.e. if I had beef, it smells a little beefy, pumpkins give it a little pumpkin odor. When you say cat pee, what does it really smell like, protein? beef? a certain vegetable? potatoes?
Kind of strange, but I just live with it.
If you google "urine smells like ammonia" you will come up with a long list of reasons, from slight dehydration to severe illnesses, so take your pick and go see a doctor if it's not gone by tomorrow.