HELP shih tzu ATTACKING pomapoo UNPRAVOKED

I had a 5 year old shih tzu and a 1 year old pomapoo. She wasn't friendly to him when they first met. She would attack him. I yelled at her and she instantly transitioned from attacking him to playing with him in a matter of 10 minutes! Now, they've been together for a year and occassionally she would go after him when my pomapoo would try taking her toy away or something. Then it grew and she began attacking him whenever someone ran down the stairs, or my dad would stand up from the table to carry his dishes in. Now, almost every night, she'll be on one end of the room and my pomapoo will be on the other and they wont be at all bothering each other, no one will move, and she'll just randomly come running over and go balistic on him. It's to the point now where I separate them and yell at her and she just ignores me! I put my hand in between them to separate them and got bitten by her. She bit SO HARD that it drew blood and has been in extreme pain the last couple of days. I don't know what's gotten into her. She just had her yearly checkup with the vet and they found she had gotten heartworms despite the Heartguard and she is on treatment for it. Could that be the issue? Is she in pain and taking it out on him? I don't know what to do. I'm going to talk to the vet when she goes in for her next treatment after the 30 days on antibiotics, but I want to get as many opinions as possible. I've never trained her to do anything. She grew up a very well behaved dog. She literally potty trained herself and never chewed up anything! She doesn't bark and she's very friendly to other people. She never had a problem with my parent's dogs. But now I'm afraid to introduce her to other dogs for fear she'll attack them too. I did have a yorkie that died a little over a year ago and when he would drop his toy off the side of my bed and jump down to get it, she would come running out from under the bed and attack him for a brief second and then stop when I yelled at her. Now she isn't stopping. I don't think she intended to bite me. I think she meant to bite my pomapoo... but she bit me hard enough to draw blood so I can't imagine what damage she could do to my pomapoo!

I think you know the answer, you never trained her to do anything. It sounds like training consists of yelling. You need to train both dogs in the basics at the very least.

Yelling is never going to solve anything. It's only likely to make the problem worse, which you see has happened.

Ultimately, I would be talking to a good behaviorist to see what can be done and I would keep the dogs separate right now. It's not fair to the younger dog to be attacked all the time. If nothing can be done to solve your older dog's aggressiveness with other dogs, it may be kinder to rehome the younger dog.