The `America`s Got Talent` host - who suffered mild kidney failure earlier this year - was treated by doctors for two clots and an enlarged right ventricle in his heart after his immune system began attacking his organs.
Speaking on his 92.3 NOW-FM show, Nick - who has nine-month-old twins Mroccan and Monroe with wife Mariah Carey - said: "I have been in the hospital since Friday, and actually, it`s quite serious. I didn`t even know. I was actually trying to downplay it a lot - even in my own mind - not go to the hospital because I was having a lot of pain in my back and I thought it was the typical kidney pain that I had been experiencing, but I thought it had been a little bit heightened....
"Because of the blood clots in my lungs, I also had an enlarged ventricle in my heart.
"My antibodies [in] my immune system were attacking my organs and it made my body even weaker."
Despite his latest health scare, the 31-year-old presenter feels "lucky" because it could have been much worse.
He added: "My doctor was like, `Luckily, you`re in shape, because most people who get blood clots in their lungs can`t breathe and they stop breathing, like, forever.` "
"I didn`t have that symptom of not being able to breathe. I mean it hurt; I had pain; when I took deep breaths it really hurt, and I told my doctors about this and they hooked me up and everything`s good now."
Nick has now been warned he has to slow down his work rate - or it will "kill" him.
He added: "My doctor was saying that my work is going to kill me - and he`s not joking.
"I think part of the reason why my health did kind of deteriorate was because I wasn`t resting a lot and I was doing too much. I was overworking myself, working out too much.
"I always took a lot of pride in working so hard and never taking a day off. But I`m learning that it isn`t the wisest thing all the time, especially being a new father. Not only wanting to spend more time with my children, but wanting to be there for my children."