The fact that she is not spayed doesn't change my answer that she needs a vet, and right away, but it does add more to my thoughts on the possible problems she is having.
An unspayed female cat is at very high risk for pyometra (a very serious infection of the uterus) and uterine cancer. Every heat cycle she has increases her risk of these diseases. Pyometra is often fatal, as is uterine cancer. Both diseases are preventable by spaying.
Every heat cycle a cat has also increases the risk of breast (mammary gland) cancer.
Going into heat every few weeks for four years is very hard on Sassy. It's hard on her body, and it is hard on her emotional and mental state.
There is still a chance that she may have a urinary tract infection. But there is also the possibility that she is starting a pyometra. Or it may simply be that she is peeing on the bed out of frustration from all the heat cycles. (the only other reason would be, if the litter boxes are dirty)
I'm glad you've been reading the link I provided, there is much good information on the whole site, the site is written by a veterinarian who has made feline nutrition her life's work.
Changing Sassy's diet should be done very slowly, that will help prevent tummy upsets that cause vomiting and diarrhea. I can help you with that and will be happy to help.
However the very first thing to do is to get Sassy to the vet.
Call tomorrow and don't let them put you off for "a week or two". Stress that you are sure she is sick because she is peeing on your bed, something she has never done before. Vets leave openings for emergencies and sick visits.
I can tell you love Sassy very much. I am curious as to why you have not have her spayed before now. She will be a much happier kitty once it is done (if she does have a urinary tract infection, UTI, the vet may want to clear that up before doing the spay), and then you can work on improving her diet, which I will help with, and you and she will have many more happy healthy years together.
PS I don't think you will be able to send me a private message, you have to have a certain number of posts first. But I will help you and Sassy as much as I can.