Justin Bieber was visibly shaken by news that his pal Lil Wayne had to be hospitalized -- and also broke some news himself ... saying he's got no beef with Floyd Mayweather.

Justin was leaving Mastro's Steakhouse in Bev Hills when our photog let him know Weezy had suffered multiple seizures in Chicago over the weekend. You can hear it in his voice ... Justin had no idea.

Then the convo turned to his other famous friend, and Justin explained what really went down between himself and Mayweather. He claims it was just about "setting boundaries."  

Not sure Floyd agrees with that take, but here's JB's story ... and he's sticking to it.