I'm so sorry to hear your little girl is giving you problems. First off I would keep being persistent and repetitive with the NO.
Also do you have a crate ? If so when she plays up give her time out in her crate, she will learn after a wile that bad behaviour results in being separated for a wile.
Thats all the advise I have to give. There are plenary of people here who have rescued chi's who I'm sure had the same problems you did and who can help you more.
The trainer sounds like a good idea.
Also ... Try keep tabs on things that set her off (if any) try write down every time she gets a little snappy and also write down the minutes that led up to it and see if you can spot a pattern might help the trainer, if he knows what sets her off.
Good luck and keep us posted and.... WELCOME