Steven Tyler -- I Did NOT Fall Off the Wagon

Steven Tyler claims he did not fall off the wagon before he fell on his face in the shower this week ... but says he understands why people might not believe him.

Tyler -- who went public about his issues with substance abuse in the past -- called in to the "Today' show this morning to talk to Matt Lauer about rumors he was using again.

Tyler responded, "I get that people think that ... It still bothers me a little, but it's something that I have to deal with for the rest of my life."

Moments later, Matt asked, "But you're clean and sober, that's not the issue?"