The 57-year-old host of FS1's "The Herd" -- one of the biggest stars in sports talk -- says he was having dinner with his wife near his home in Southern California when he felt a sharp pain on the right side of his chest.

So, Cowherd phoned a friend who raced over, picked him up and sped Colin to a nearby hospital for emergency treatment.

Cowherd says he was in "incredible pain" and was understandably scared -- thinking he may have come down with COVID.

Doctors told Colin he tested NEGATIVE for coronavirus twice during his hospital stay -- but noted they found a small blood clot in his right lung.

The clot was limiting Cowherd's breathing so doctors admitted him to a room and used morphine to control the pain.

"I was pretty much knocked out with morphine, a narcotic, for 16 to 18 hours," Cowherd said.

Now, doctors are trying to figure out WHY Cowherd suffered the clot.

"I'm waiting for the blood work," Colin said ... "I have no idea why it happened. I have been doing a lot of skiing at high altitudes, traveling, that could be it. We don't know. We’ll find out in a few days."

Cowherd says he's already feeling better -- noting he went on a 30-minute walk on Tuesday ... "which is amazing considering where I was 48 hours ago."

Cowherd says the goal is to return to his show as soon as possible -- but the focus is on his recovery ... and he shouted out FS1 and iHeartRadio for being incredibly supportive during this scary time.