9:50 AM PT -- Sources connected to Joe tell TMZ, he's been having "breathing issues for the last few weeks and it came to a head last night." We're told it involves congestion in the lungs.

Our sources say Joe is still in the hospital but "he's awake, doing well and could be released as early as Sunday afternoon."

Joe Perry was wheeled out of Madison Square Garden on a stretcher Saturday night after performing with Billy Joel.

Eyewitnesses tell TMZ, Joe had just finished "Walk This Way" with Billy Joel onstage and went back to his dressing room. A source connected with the concert tells TMZ, Joe collapsed and "looked terrible."

Paramedics rushed to the scene and worked on the 68-year-old for around 40 minutes in his dressing room. We're told they put a tracheal tube down his throat to clear the airway. There was panic in the dressing room, and one eyewitness says "Joe did not look good."

Paramedics put the famed Aerosmith guitarist on a stretcher, gave him oxygen and rushed him to a hospital.

We're told Billy Joel was still on stage in the final minutes of his concert and was unaware of the emergency. Paramedics and security had to clear a path to get out of MSG because there were so many people around.

We do not know Joe's condition.

We got Joe out back in August and he was in good spirits.

Aerosmith has been planning a Las Vegas residency for 2019. Joe and Steven Tyler have had a volcanic relationship, but they have been good lately and have been looking forward to their Sin City concerts.

Story developing.