So THAT's why he's always getting an IV drip! We finally know the reason Justin has suffered from depression and anxiety the past couple of years and why he's been taking vitamins intravenously as of late ... because he's battling Lyme disease!

Lyme is contracted from an infected tick and can cause rashes, headaches, fever and fatigue. Those suffering from the disease can recover, in most cases, with antibiotic treatment.

Justin makes the admission in his upcoming doc, set for release Jan 27. The singer explains that those pix of him shot back in 2018, crying, with his head in his hands -- and all those disturbing, sad pix since -- we a result of him suffering from an undiagnosed condition. Apparently it took months for doctors to figure out exactly what was ailing Bieber and during the interim, he was breaking down mentally and emotionally. With the help of Hailey, he pushed through, got a proper diagnosis, and started getting treatment. The cocktail of medicines he's on has caused him to break out -- but no worries, JB's not concerned with appearances, he just wanted to get healthy. But getting there has taken a toll ...

Also set to be revealed in the biopic is that after his upcoming tour, Justin and Hailey plan to start a family!!!