Courtesy of Amanda Shoultz

(DALLAS) -- Amanda Shoultz said she spent most of the past year trying to lose weight after noticing that her stomach kept getting bigger.

"I started working out more. I was dieting more and, oddly enough, I was losing weight but I was gaining inches in my stomach," Shoultz, 29, told Good Morning America. "For the longest time, I thought, 'Oh, I just must gain weight in my stomach.'"

When Shoultz, of Dallas, Texas, went to her annual checkup with her primary care physician in February, she said she was shocked at the number she saw on the scale.

"I remember telling her, 'The next time you see me I'm going to be 10 pounds lighter,'" said Shoultz. "I just assumed it was my fault. That I had done something wrong."

Shoultz's blood work from her doctor's appointment came back normal, so she said she kept changing her diet to see if she had an allergy that was causing her stomach to bloat.

"I gave up all dairy products because I thought it was a lactose allergy, and nothing changed. Then I gave up gluten. I love bread but I was willing to do anything," she said. "That didn't work so then I gave up meat. None of that helped."

Shoultz said she did not feel any pain beyond the discomfort of her stomach getting so much bigger than usual.

By August, nearly eight months after first noticing the growth, Shoultz was referred to a gastroenterologist by a colleague at Baylor Scott & White Heart and Vascular Hospital, where she works in public relations.

"By the time I saw my GI [gastroenterologist] doctor, my stomach was hard as a rock," she said. "My mom said you could have punched me in my stomach and broken your hand it was so hard."

After several tests that did not find anything definitively wrong, Shoultz underwent a CT scan in late September.

"About four hours later, I got a call from my doctor who told me I had a 33-centimeter tumor in my abdomen," she recalled. "And within two days of that, I was already meeting with the surgeon."

Doctors discovered that Shoultz's tumor was cancerous. She was diagnosed with liposarcoma, a rare form of tumor that, in most cases, does not present any symptoms until the tumor grows large and invades other organs or tissues, according to the National Institutes of Health (NIH).

In Shoultz's case, the tumor had formed around her right kidney and adrenal gland, but she experienced no symptoms to indicate that cancer was growing inside her.

"I'm 29 and otherwise perfectly healthy," she said. "I had no other symptoms other than my stomach."

On Sept. 27, Shoultz underwent a two-hour surgery to remove the tumor, which doctors discovered upon removal weighed 17 pounds.

Doctors also had to remove Shoultz's right kidney and part of her adrenal gland. Because the cancer had not spread to other parts of her body, she did not have to undergo further treatment, like chemotherapy or radiation.

"Once I left the hospital, my stomach was back to normal," said Shoultz. "Now I'm just eating all the food that I missed when I gave it up for a year in order to put some weight back on."

Shoultz said she is sharing her story publicly because she wants other people, particularly women, to know the importance of knowing and listening to their own bodies.

"I knew that something was wrong because I've always had a hard time gaining weight," said Shoultz. "When I was getting so large in my abdomen and I couldn't control it, that's when I knew something was off."

"We preach it at the hospital, don't die of doubt," she said. "No one else is going to need to fight for you, so fight for yourself and find a care team that is going to care for you through the journey."

It's a message echoed by Dr. Robert Mennel, an oncologist with Texas Oncology at Baylor University Medical Center in Dallas, who is treating Shoultz.

"There is a whole group of these sarcomas that tend to occur in younger people and a lot of times they have symptoms that are sort of very nondescript symptoms," he said. "If somebody comes in and they have some abdominal discomfort and it's somebody who's in their 20s or so, most physicians think that's probably not much and it will pass."

"We can't do scans on everybody, but if you really feel that something is wrong, just be persistent to get this evaluated," added Mennel. "And make sure you're going to somebody who has experience and really knows what they're doing."

"The takeaway would be that if you feel that something is not correct, see a physician or health care provider to let them evaluate it," he said. "And if you really feel that they're not evaluating it, or if you really feel that something's wrong, pursue your desire to get it looked at and worked up."

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