I'm so sorry all this is happening. Poor Peekaboo. I hope she'll be able to go potty soon. If she doesn't get better soon though I would take her to a cat specialist for a second opinion. In my experience most vets have the most experience in treating dogs and are a bit clueless about cats. Specialists charge a bit more but it's worth it. An ultrasound may be in order to see if there is something somewhere else.
I hope the reason the vet didn't find anything was because it's no longer there. Cats can easily get obstructions from hairballs and that's just something that happens. I don't think poisoning would show up as an obstruction. If Peekaboo does pick up kitty litter with her mouth though chances are that some does get swallowed and that is not good. It expands in the stomach and can get lodged there. I had a foster dog that ate cat litter and got an obstruction. She didn't intend to eat the litter, it just came with the poop she got into, so I don't think she could have eaten that much but it was enough to cause a problem. I suspect that it added up over time. Maybe the same thing happened with Peekaboo. If she will accept it you may want to try using S'wheat Scoop litter as that is safe to swallow. It actually works pretty well. I think World's Best Cat Litter is also safe to swallow and that stuff is also really good
Please keep us updated on Peekaboo's progress.