Diana Zepeda, 34, says she initially thought she had simple stomach problems, but got a much scarier diagnosis during a trip to the doctor's office

Diana Zepeda, of Washington, D.C., says she initially thought her stomach pains were due to an unhealthy diet. But a trip to the doctor resulted in a much scarier diagnosis for the 34-year-old.

“I thought I could eat anything and have a stomach of steel. I was getting what I thought was random food poisoning, but kind of often. A lot of gas, cramps and diarrhea,” Zepeda tells PEOPLE. “I just thought I had one of those stomach bugs and it would just last a couple of days.”

She adds: “I thought I could fix it with diet, I thought that was my issue. So I did a whole month where I cut out grains and dairy and sugar. And I’m like, ‘This is what my body needs and it’ll go away!’ ”

Still, over about three months, Zepeda’s symptoms grew worse. By January 2017, Zepeda was suffering diarrhea every day and even found blood in her stool, she says. She decided to meet with a gastroenterologist, and tests showed Zepeda had E. Coli. But the prescribed antibiotics for the illness didn’t help.

Credit: Diana Zepeda

“First I was relieved that whatever it was could be cured with five days of antibiotics. I thought it was over … happily ever after. But that wasn’t the case unfortunately,” she says. “I was dreading what ever the actual diagnosis would be.”

Soon, a doctor scheduled a colonoscopy for Zepeda. But, while prepping for the procedure, she suffered severe vomiting, stomach cramps and nausea. So, doctors decided to give Zepeda a sigmoidoscopy (a partial colonoscopy). That’s when they made a shocking discovery: there was a tumor blocking Zepeda’s colon.

Doctors diagnosed Zepeda with stage 4 colon cancer.

“After the initial shock and disbelief, my first thoughts were, ‘What did I do wrong to get this?’ Because I think everyone thinks of colon cancer as an old people’s disease,” Zepeda says. “I thought, ‘Was it all the microwaved Pop-Tarts I ate in college? Or all the microwave Lean Cuisines? Like, what did I do to cause this?’ ”

The cancer spread to her liver and Zepeda spent the next six months undergoing chemotherapy and a series of surgeries. She says the experience left her feeling “completely depleted.”

Credit: Diana Zepeda

“I didn’t have the strength to walk for almost an entire month. I was bedridden. The hardest part was definitely chemo … After the very first treatment, I was ready to quit. I felt really depressed and really isolated. I was like, ‘I can’t do this. I’ve already gone through enough. I can’t do six months of this.’ ”

But she did. Zepeda celebrated underwent her final treatment on Thursday. To celebrate the big day, she and her husband, Alexander Sweeney, dressed in formal wear to the hospital.

Credit: Diana Zepeda

“I was really distraught at the beginning of it, thinking I couldn’t do it,” she recalls. “Then I went, just like, ‘I need to finish this, what ever it takes.’And here I am, I just finished today!”

Credit: Diana Zepeda

Although Zepeda is set to undergo another surgery soon, and it will be a few years before she’s officially in remission, Zepeda says she’s excited to get back to living her life.

“I really miss traveling and just being busy with friends,” she tells PEOPLE. “I’ve been sitting everything out and I’ve had FOMO [fear of missing out] for like nine months. I’m excited to go back to normal as much as I can.”

Now, Zepeda is sharing her story to raise awareness about “young onset” colon cancer. Researchers with the Colorectal Cancer Alliance are looking to determine why more young people are suffering from the illness. People under the age of 50 are at least four times more likely to be diagnosed with colon cancer than they were in 1990, according to the CCA, citing the American Cancer Society.