Hip Hop Site to Chad Johnson -- Go Ahead, Sue Us Over Your XXX Vid

Chad Johnson's sex tape woes keep getting worse ... the former NFL star is suing two sites for illegally posting the video online -- and now one of 'em wants Chad to know they're not gonna bend over and take it.

TMZ broke the story ... a XXX video recently popped up starring Chad. He fired off a cease and desist letter to the sites that posted it -- including WorldstarHipHop. The sites took the skin flick down, Chad -- who wanted a pound of flesh -- filed a lawsuit.

A source close to Worldstar tells us ... when it came to acquiring the video, it did everything by the book after being assured it was being sold by the legal owner. Therefore, Worldstar contends Chad's lawsuit is baseless.