Hello, my name isXXXXX and I have over 20 years of experience as a veterinarian.
I am very sorry to hear about Buttercup having a pink color to her urine.
I am glad to hear that she is feeling otherwise.
Pinkish urine can signify a urinary tract infection, crystals or stones in the urinary tract or a mass in the urinary tract. It can also indicate a clotting disorder, either due to ingesting rodent poison or liver disease as the liver makes clotting factors, tick borne diseases or autoimmune diseases (body attacking itself). Other possibilities are a systemic infection (throughout the body) such as from a bacterial infection or an inflammatory condition called interstitial cystitis.
I suspect her not using her litterbox consistently is related to her pink urine. Cats that feel discomfort when eliminating will often have discomfort and choose to go elsewhere hoping not to feel discomfort.
Because she seem to feel fine otherwise I don't think this is an emergency today. But I do recommend she see her veterinarian for an examination and a urinalysis with a culture to look for signs of infection or crystals.
If your veterinarian feels a thickened bladder upon examination radiographs of her abdomen should be done. An ultrasound would also allow them to check for radiolucent calculi (not seen on regular radiographs) in her bladder or kidneys, a mass in the urinary tract and evaluate her bladder wall for signs of interstitial cystitis. Interstitial cystitis is a poorly understood inflammatory condition that causes thickening of the bladder wall, discomfort, a poor mucous layer on the bladder wall and bleeding of the bladder lining. It seems to worsen with stressful conditions. Your veterinarian may also want to check blood tests for signs of internal organ disease.
In the meantime I recommend increasing fluid intake as much as possible via canned diets, a kitty drinking fountain and offering low salt chicken broth or tuna juice to drink.
In short there are several things that can cause pink colored urine beyond crystals and infections in the urinary tract. Sometimes extensive diagnostics need to be done to pin things down.
Hopefully the information I have given you will help find the cause of Buttercup's problem.
Please reply should you have any further questions.