Joey Fatone says he knew his friend Chyna was "having some trouble" recently but didn't know how bad her issues had become ... telling TMZ Sports they had just made plans to hang out.
Fatone and Chyna have known each other for years -- and the NSYNC member says he was sad to learn of her passing this week ... especially since he just had a conversation with her last week.
"I know she was having some trouble and stuff like that but didn't know [it had gotten] that bad."
Chyna had struggled with substance abuse for years -- and officials are investigating her death as a possible overdose.