Harper suffers from an overactive bladder and told flight attendants she had to use the restroom, she said. When they told her to return to her seat, she said she tried to explain the urgency of the situation.
"I said 'I'm going to need to use the restroom or I'm going to need a cup,'" Harper told NBC's Kansas City affiliate. "[They] didn't understand that I don't have any control over the situation."
That's when Harper said the flight crew did something surprising: They handed her a cup.
The St. Luke's Health System nurse told NBC's KSHB the cup was so small, she had to ask for a second one.
"A completely humiliating situation," said Harper. "There was people all around ... you know, strangers."
Harper said she alerted the flight crew of her condition, an overactive bladder she's lived with her entire life.
"I don't know if they just didn't understand that, you know, I really did not have any control over the situation," said Harper.
Harper filed a complaint with United as soon as the plane landed, she said in a Facebook post. She has since also filed a complaint with the Department of Transportation.
On Wednesday, United issued a statement denying that a flight attendant encouraged Harper to urinate into a cup.
"Customer safety is always our first priority. Initial reports from the Mesa Airlines flight attendants indicate that Ms. Harper attempted to visit the lavatory on descent and was instructed to remain seated with the seat belt fastened per FAA regulations," said the United Airlines statement.
"At no point during the flight did flight attendants suggest that Ms. Harper use cups instead of the lavatory," the statement continued. "We have reached out to Ms. Harper to better understand what occurred and we continue to review what happened."
It's just the latest in a string of controversies involving United Airlines staff. In late March, two teen girls and a young child were initially barred from their flight because they were wearing leggings. The child was allowed on the plane after agreeing to put on a dress, and the story exploded on social media as an example of the way women's clothing is policed.
In late April, a giant prized rabbit died on a United flight to Chicago, prompting the animal's owner to retain an attorney. And just this Monday, the airline faced heat for a video shot on a customer's phone that showed a United employee canceling a man's flight because she was angry that he was recording her.