Do some deeper research. Read open sources. In answer: Flynn briefed the DIA before and after contacts with Russia. The man is known as the best intel agent in our military. Strictly professional. Think spook, not greedy self-promoter as portrayed in media.
OK, so he apparently was briefed before and after attending the December 2015 big anniversary gala for the state-run "news" (propaganda) outlet Russia Today (RT). It seems clear to me that he was invited there by the Russians with the intent to at least make it APPEAR as if he were working with them, in this highly sophisticated, elaborate, but typical KGB/FSB-styled hoax.
Dec 2015, btw, is the same time frame when Putin was heaping praise on Trump, in, IMO, an attempt to make it *appear* as if Trump was his guy, when in reality, he was anything but.
In any case, it wasn't only the Russia Today anniversary gala. Flynn appeared as a regular guest on that Russian fake news network. Then maybe that too was him being used by the Russians, although he would have been very naive to have fallen for a ploy like that.
Good article on this precise subject here: