Tiffany Reyes had just gotten back to her seat from the bathroom and was about to buckle her safety belt when Hawaiian Airlines Flight 35 dipped.

Key points:

• Unexpected turbulence hit a plane flying from Phoenix to Honolulu
• 36 people received medical treatment, 11 were in a serious condition
• The National Transportation Safety Board will investigate the incident

In an instant, Ms Reyes found herself on the aisle floor, staring up at caved-in ceiling panels and a cracked bathroom sign that was hanging.

"I asked everyone around me, 'Was that me?'" she said.

"They said I had apparently flown into the ceiling and slammed into the ground."

Ms Reyes, 40, was among 20 people on the flight — passengers and crew — taken to hospital after turbulence struck their plane without warning as it flew from Phoenix to Honolulu Sunday.

Director of Honolulu Emergency Medical Services Jim Ireland said 36 people received medical treatment for bumps, bruises, cuts and nausea and 11 were in a serious condition. 

Ms Reyes initially thought something had hit the plane and it was crashing. She briefly thought they were going to die.

"That's the most terrifying experience I've been through in my whole 40 years of life," she said.

She said others had it worse as she saw a woman walk off the plane with gashes on her head and blood on her face and clothes.

An ambulance took Ms Reyes to an emergency room and after five hours and various tests, she and her family went home. 

She had a headache which began to fade Sunday night. But the left side of her body started to ache.

"I can't even move around in bed," Ms Reyes said.

"So I have to sleep right on my back without even moving."

The National Transportation Safety Board (NTSB) said it was investigating the incident.

The full flight had nearly 300 people aboard, many travelling to Hawaii for the holidays.

Hawaiian Airlines chief operating officer Jon Snook said such turbulence was isolated and unusual, noting the airline had not experienced anything like it in recent history.

He said three flight attendants were injured and there was some internal damage done to the aircraft.

He said some of those injured were not wearing seatbelts despite the sign being on. 

The airline was aware of the forecast for thunderstorms and unstable air and weather conditions, but had no warning that the particular patch of air where the turbulence occurred "was in any way dangerous," Mr Snook said.

The NTSB investigation will address what the passengers and crew were doing at the time and how far the plane dropped, he said.