Ray J was rushed to the hospital Thursday after suffering a medical emergency in the middle of a live stream ... and it sounds like there was a problem with his heart.

Ray was streaming from Fab Factory when he started having health issues, prompting someone to call the fire department and an ambulance.

His mother was also on the stream with him.

Video from the stream shows paramedics tending to Ray as he lies shirtless with medical equipment attached to his chest.

We're told Ray was taken away by ambulance with what sources describe as heart-related issues. The call was strictly medical, and we're told there was nothing unusual or suspicious going on.

The health scare comes as Ray has been pushing himself through "Rayhab" -- his 180-day streaming subathon -- and we're told he's been staying up for long stretches while streaming.

As TMZ previously reported, Ray has dealt with serious heart issues this year ... including a January hospitalization for pneumonia and heart pain, and another hospital stay in May after his heart rate abruptly slowed following his fight with Supa Hot Fire.

Story developing ...