“I had no idea that this could happen.”

A Massachusetts mom is issuing a PSA for parents after a frightening day at the shore.

“I’m still shaking,” Heather Cassini tells TODAY.com.

In a now-viral Facebook post, Cassini, 40, shared that she was visiting New Hampshire’s Hampton Beach on July 4, when her 10-year-old son, Declan, announced he was feeling nauseated.

It was a hot and humid day, and Declan had been in the ocean playing. Cassini wasn’t concerned, and figured he needed to rest in the shade.

“I thought the breakfast was just too much for the waves and he just needed to lay down,” Cassini wrote. Then, Declan became “disoriented,” and collapsed, landing directly on a dozing sunbather.

Declan was able to pull himself up, but moments later, he stumbled and was back on the ground.

“I’m pregnant and frantically trying to pick him up,” she recalls. “He’s going in and out of consciousness and vomiting. And he was so pale.”

That’s when Cassini says a group of women swooped in to help. One flagged down a lifeguard, another called 911.

“God bless all the people around us. There were so many nurses,” Cassini says

Paramedics determined that Declan had hypothermia from the 52 degree water.

“I had no idea that this could happen,” Cassini says. “You think about sunburns and dehydration and all the things that can happen in the water, but I never considered cold shock.”

She says that Declan has made a full recovery.

Water below 70°F can cause hypothermia even on a balmy day, according to Dr. Christopher Kelly, chief of pediatric emergency medicine at NewYork-Presbyterian Brooklyn Methodist Hospital.

“Temperatures drop relatively quickly in kids, so that puts them at a higher risk for hypothermia,” Kelly, who did not treat Declan, tells TODAY.com. “As their temperature drops lower and lower, they start to get a little confused, and you might notice some slurred speech.”

If you suspect your child is experiencing signs and symptoms hypothermia, Kelly says to seek medical care immediately. It’s important to rule out other causes such as low blood sugar.

“You want to dry them off with warm — or at least dry — towels,” he says. “And you want to keep their heads covered because kids lose a lot of heat from the top of their heads. Other than, it’s really just kind of that gentle warming until their body temperature comes up.”

To prevent hypothermia, Kelly says to pull your child out of cold water for periodic breaks — even if they insist they’re fine.

“Kids just want to have fun and don’t care if they’re cold,” he says.

Cassini’s Facebook posted has been shared 12,000 times.

“This exact same thing happened to my son on a beach in Maine a few years ago. Thankfully there were a lot of nurses on the beach that day but we ended up going to the ER just to be sure. Thank goodness your son is ok. They bounce back so quick. Us moms…..not so much,” one person wrote.

Added another, “This same thing happened to me when I was 10 years old. It was super scary for me and all involved. I’m very glad your little boy is okay. And thank you for sharing your story so more parents and caregivers can learn from your experience.”