feel sick with nerves

Hi,

Do you know what feel sick with nerves mean?
and in the sentence 'Not only had his girlfriend gone off with his best friend, but he didnt get the promotion he wanted,' I would like to know if You always invert the subject when you use Not only.
Could you please help me?
Thank you

You feel sick with nerves when you are waiting to go onto a stage for the first time, or when waiting for the doctor to tell you the result of tests, or when the exam results are due but are late.

Yes, you have to invert when using not only.