If you are learning English you would no doubt say that 'says' is the third person singular of the verb 'to say' and it therefore follows a singular noun or pronoun (John says, she says, the book says).
In fact, it is reasonably common in colloquial English to say, or hear, "says you" or "says I". The Standard English equivalent would be "You said" and "I said". This construction is in the Oxford English Dictionary with this explanation: the 3rd sing. pres. is often substituted colloq. for the pa. tense said. Hence, in vulgar speech or jocular imitations of it, says I, says you = ‘said I’, ‘said you’.
Because it is so colloquial, 'says' is often written 'sez'. The phrase is usually said or written in the order 'says you', 'sez I' etc and not the other way round.
These phrases are usually used when talking to a third party, or writing on an informal internet forum, and usually refers to something that you did not say but might have done. It is difficult to explain exactly when it is appropriate to say them, but here are a few examples I've found on the internet (written as found). These are all from Google Ireland pages. That's because my Irish relatives say these phrases a lot, so I thought I might get good examples from Irish sources.
"Anyhow, Im an FG supporter ('course you are sez you)" (a comment on this blog).
"Therefore, points out the Minister, the degree to which VAT is passed on should be less than the full rate of 13.5 per cent. Hmmmm, sez you!" (from this newspaper article) .
"The difference between this and other networking sites is that it's not just about "social" networking. Businesses, government bodies, NGOs and groups of all kinds are also welcomed. "So what?" sez you" (a contribution on this social networking forum).
"This week she gave the nixers to Aidan Power, Baz Thingummy and Dustin the Turkey. And why not, sez I -- these are straitened times and the least you can do is help out your RTE pals" (from this newspaper column)
"I'm not says I...I'm fine!" (title/slogan here)
As you see, there is often an ironic undertone to the comments, if not downright sarcasm.