Jissel Rosario, 12, said she woke up one morning in April with a strange pain in her left leg. 

“Like someone was... I don't know, punching my leg or stabbing my leg," she said. 

She hadn’t hurt herself in any way -- no cuts, no bruises, no sign of anything. After all, her mom had kept her largely inside their Newark apartment for weeks, fearful she could contract COVID-19. 

Jissel’s leg started to discolor, then swell. Her mom, Alma Cruz, rushed her to the hospital. Jissel had severe clotting in her left leg, doctors at Newark Beth Israel Medical Center said. 

The clot would prove nearly fatal. It caused her heart to stop but a team of doctors was able to save her life. At first, doctors didn’t think it was related to the coronavirus. Jissel tested negative for the virus three times. But then, she tested positive for the antibodies. 

Dr. Marc Cohen, chairman of the Department of Medicine at Newark Beth Israel, said her severe clotting is likely a delayed reaction to the virus. 

"The infection occurred before and you don't face the music until after,” he said. “So that's just another feather in the cap of COVID being a horrific nightmare.” 

It's not clear whether Jissel falls squarely into the category of multi-system inflammatory syndrome in children. The rare condition has sickened 157 children in New York and 19 in New Jersey. New Jersey’s health department would not confirm whether Jissel’s case is among those reported in the state. 

The CDC says the condition is linked to COVID-19 and causes inflammation of the blood vessels and fever. Although it has affected a small number of young patients, the syndrome has rattled early assurances that children were largely unaffected by the virus.

Dr. Cohen said Jissel never had pneumonia, a fever, or any signs of an acute infection. She was a healthy child with no known underlying medical conditions. Unlike the inflammatory syndrome that impacts a child’s arteries (responsible for pumping blood from the heart to the rest of the body), Dr. Cohen said Jissel’s veins were affected. 

One of the common threads among some of these cases and Jissel’s: The body is still reacting even after the acute infection is gone.

“It’s more of a patient’s immune system,” said Dr. Jennifer Owensby, interim medical director at Bristol Myers Squibb Children's Hospital at RWJ University Hospital. “For whatever reason it keeps fighting; it keeps attacking. And, it is attacking the patient’s own body.”

Dr. Owensby said her hospital has treated six pediatric patients that may have multi-inflammatory syndrome. Half tested positive for COVID-19, the other half tested positive for the antibodies.

“During the initial COVID pandemic, we absolutely saw pediatric patients with respiratory syndrome, respiratory failure, on ventilators, pneumonia,” she said.”Now over the past two to three weeks, we are seeing the post-infectious syndrome.”

Dr. Owensby added she hasn’t seen clotting in her pediatric patients testing positive for COVID-19 or the antibodies, but she has noticed an imbalance in the chemical makeup of their blood that makes it more likely for clots to form. 

Blood clots have been identified in adult COVID-19 patients, according to reports and medical studies. But, generally, blood clots in healthy kids are not common. 

Jissel Rosario and her mom, Alma Cruz, after a day of therapy on May 19, 2020

“It is never common for a 12-year-old to out-of-the-blue develop a blood clot in the leg,” Dr. Cohen said. 

Alma Cruz said when she first took Jissel to an urgent care clinic, they thought she had cellulitis, a skin infection, and prescribed antibiotics. 

They didn’t work. Jissel’s leg turned purple; her toes red.

Cruz, 33, rushed Jissel to the hospital. Jissel said she was scared to go, worried about catching COVID — or losing a limb.

"My mom always calmed me down and told me everything was going to be OK," Jissel said. 

At Newark Beth Israel, doctors found the veins in Jissel's leg plugged up with clot. Her leg was two to three times its normal size. Dr. Cohen said during an invasive procedure to clear out her veins, the clot moved to her lungs and stopped her heart.

Jissel underwent CPR for almost an hour. A machine took over circulating her blood and a ventilator helped her breathe. Doctors fed blood-clot-dissolving medicine into her body through catheters inserted in her neck and groin.

Jissel said she doesn’t remember much. 

"When I woke up, I remember I was trying to move, but I couldn't. And so I got super frustrated and I started crying,” she recalled. “And then I got thirsty. And I asked my mom for some water, but I couldn't drink anything because I had the tubes in my mouth.”

She does remember the weeks she spent recovering, making up songs with her mom, singing Spice Girls, the Backstreet Boys. There were also the health care workers who played UNO with her and made slime. And being able to hang out with her mom all the time, being silly.

When she was discharged last week, the staff cheered her off. They gave her balloons and blew kisses.

Jissel is still using a walker to get around and comes back to the hospital every day for therapy. Her foot is bandaged and she still feels some pain. She can't wait to "walk normally, run around my house and annoy my mom." 

"I look forward to that, too," Cruz said.