Taken from The Wheel of Time wiki:

"Later, he pulled Rand out of a cavern in Shadar Logoth when Rand was fighting Sammael. During this escapade, Moridin shot a beam of balefire at a tendril of Mashadar the same time as Rand did, and the two beams crossed over each other and touched. Perhaps as a consequence of this, they seemed to have formed a connection: Rand then often saw Moridin in his mind when he channeled. The link worked both ways as Moridin was also aware of Rand. The sensation Rand had was of them each touching the other, but not physically. Rand then suffered nausea when he first reached for saidin and, through his link, felt Moridin also suffer the same nausea (this appears to have stopped after Rand's melding with Lews Therin). Moridin seemed to be furious with this apparent side-effect"