yorkie suddenly aggressive towards shih tzu

[mscat]

We have had Suzi over 4 years now, and brought home a shih tzu puppy a year later, they have always gotten along well, however, yesterday uzi our yorkie attacked Miely over a bone/bully stick, I always give them one each, and do not always give them one because Suzi gets hers first, then runs over and grabs Miley's bully stick, or she will hide one and take miely's. So i gave Miely a different bone alltogether, so she could have something to chew, suddenly a fight broke out between them, I picked up Suzi , took the "extra bone and they were ok, but I decided to give Miley a bone in my room and shut the the bedroom door with Miely so she could have one too, but when I checked on her she did not even want the bone anymore.
This happened in the afternoon, later on in the night I was sitting on the couch , called Miley over to pet her, and Suzi jumps on the couch and they start fighting all over again , I held them back between each other, and called my son to help. Whil waiting for my son both dogs were growling and showing teeth , My son took Suzi in his room , it is his service dog, and Suzi is better with him. They have not ever been totally aggressive against each other like this. Suzi has always been the dominite dog, over Miely who is mild mannered and laid back. Not sure what to do? Hopefully these are isolated incidents. But, it is really frightning to se this happening. Especially after the use to play together all the time .
2 weeks ago both dogs went in to the vet, got their vaccines updated, no unecessary ones, and microchipped, blood tested for any parasites, and heartworms, and are now on Sentinel given once a month. No reactions happened. I was also able to get them their city licensed free for life due to the dogs being service animals.
the vet did tell me Suzi needs a teeth cleaning, and at the same time to get spayed. I had spent close to $400 the last vet, which is not too bad for 2 dogs. They were both in heat when I took them to the vet, so the vet advised me to wait until after the het cycle to get them spayed. He told me to get Suzi done first. I know from reading on here to always get a blood test before a spay, and teeth cleaning, total cost will come to around 400 the spay is less then the dental and blood work. Will Suzi calm down after that? is it her tooth that is making her more aggressive? one of her back tooth is not good, but the vet did not say if it needs to be pulled .
Suzi is showing aggressive behavior though and this is what upsets me the most. I do not know if a dog "feels" jealous or if she is in pain, or what. Their fights are quickly fused out, and they are not out for blood. Still it is a concern. Both dogs are equal in weight and size. Miely has a lot more hair though so Suzi can't really sink her teeth into her. Can anyone give me advice?