I honestly don't think you will have anything at 600 mg because it's not even at therapeutic levels. I have cold turkey 900 mg before and experienced an episode about 2 weeks after I stopped, which went away when I started the dose again.

Chest hurting is usually anxiety.

I think you should have talked to your doctor about quitting the lithium so you could try something else instead of quitting it. Do you do/take anything for anxiety?