I work in California and I sued a client for nonpayment and I won the case. Now the client is suing me for $1900 for time spent going over my work and claiming that 80% of my work was wrong. I have over 20 years experience in my field. Can I counter sue for so much stress I had to see a dr. (not covered by my insurance) and also defamation.

No, you cannot unless the client did this deliberately, without cause, without justification, knowing that the lawsuit was false.

It becomes defamation when you are harmed - can you prove monetary loss?

As a rule of thumb Court filings are exempt from slander/defamation, as is Court testimony.

If you could sue for stress I'd be a billionaire by now. We really can't answer the rest of your issue since we have no clue what you did, how well, etc... Looks a judge will be determing this shortly. Get a good lawyer and have him/her state this is haraassment due to a previous suit and is frivolous, worth a shot. I am very much surprised this issue was not raised by the client during your suit and the next judge will wonder this too.

Thank you for your answers. I am a bookkeeper and this job was done online. The client has access to all my work and she still hasn't paid me even though I won the case. She is now suing me out of spite for $1900 for time spent going over the work I did. I no longer have online access so I don't know what they are doing.

I repeat what I said earlier and I'll ask a question.

Can you PROVE the lawsuit against you is spiteful OR is there a possibility she was unhappy with your work.

I don't understand why she didn't simply countersue if it's the same matter.

She is suing me for her time, the president's time and CPA time going over my work. I can't prove my work as it was done online and I have no access nor do I know what I did wrong if anything.
She is mad that I sued her for nonpayment and I won.

I know what you are saying BUT you are asking if you can sue. No, you cannot if you cannot prove what you are claiming. I understand why you can't but it pretty much ends there.

You can always counterclaim with your thinking and see if the Judge goes for it -