Michelle Whiteid and her family sat with NewsNation's Brian Entin for an interview that's airing in full later Tuesday -- and she says Riley was texting her the night of his disappearance, where she says he told her his rum and coke did "not taste good."

Michelle says Riley claimed the beverage tasted like barbecue ... reiterating it was not an enjoyable cocktail. Michelle says she told Riley he probably shouldn't be drinking it then.

This exchange has left Michelle seemingly convinced her son's drink was spiked with something at Luke Bryan's 32 Bridge bar in Nashville -- the last place he was seen before wandering off into the streets, where he vanished.

She adds ... "Maybe there was something in it that shouldn't have been."

Remember, the bar previously claimed Riley was only served one drink and 2 waters before being escorted out of the bar.

Of course, this isn't a smoking gun by any means -- but it does lead to the possibility he was roofied ... as Nashville's nightlife has been the target of multiple drugging attempts in recent years. It's definitely a thing, and it's not outside the realm of what might've happened.

The other possibility, of course, is that Riley was in fact heavily intoxicated that night ... and showed up to Luke's already drunk. The bar itself has said they booted him for that exact reason ... claiming he was acting up. They say they have surveillance video to back it up too, which they have yet to release.

While Riley's cause of death appears to be accidental, per the autopsy report -- the toxicology report is still pending ... and several questions remain unanswered for now.

Riley's family has consistently said they believe something else happened to their loved one that night ... refusing to accept the police's investigation, which determined there was no foul play. The one new thing Riley's mom is expected to do in this interview ... slam his friends.