Taylor Muhl has chimerism, a rare disorder in which a person has two sets of DNA — in Muhl's case, she lives with the DNA of her fraternal twin

Ever since Taylor Muhl was a little girl, she longed to have a twin sibling. The singer and model would often ask her mother whether she was a twin and became obsessed with dressing just like her friends. Years later, Muhl learned that her wish had already come true.

The Los Angeles native grew up believing a discoloration on her stomach was simply a birthmark. And after years of battling autoimmune issues, Muhl went to a doctor who diagnosed her with chimerism: a rare disorder in which a person has two sets of DNA — in Muhl’s case, she lives with the DNA of her fraternal twin whom she absorbed in her mother’s womb.

“My first reaction was shock,” Muhl tells PEOPLE of the moment she learned about her condition almost eight years ago. “Then my second reaction was sadness because I was like, ‘Oh my gosh! I could’ve gone through life with a twin, with a sibling that hopefully I could’ve been close to.’ There was some sadness.”

Credit: Courtesy Taylor Muhl

Muhl fused with her sibling in the womb, unbeknownst to her mother who opted for a “natural” pregnancy and never had an ultrasound, Muhl tells PEOPLE. Now, she carries her twin’s DNA in her own body, with two different blood streams and immune systems. Thus, Muhl’s body is constantly working to fight off the twin’s cells, as it recognizes them as foreign, Yahoo reports.

“I actually felt relief. I felt freedom because for the first time in my life I knew why my stomach looks the way it does. This is why I’ve had so many health challenges,” the model says of coming to term with the news. “I felt like for the first time I could really breathe. Prior to that, every doctor said my stomach must have just been a birthmark … Finally, this is making some sense.”

Muhl says her mother, who didn’t have more children, was saddened by the news, viewing the situation as “losing a child.”

Credit: Courtesy Taylor Muhl

“I think, for her, it was really sad just to think about, ‘Wow, I could’ve had two babies.’ ”

Now, Muhl focuses on living a healthy, active lifestyle to cope with her condition. Although she has known about her disorder for several years, she only went public with the news last year. Muhl says she kept her condition private —and hid her stomach — in fear of losing opportunities in the entertainment industry.

Credit: Courtesy Taylor Muhl

“This has probably been the most freeing year that I have ever lived in my entire life. What was so hard for me was pretending that I wasn’t sick and it’s something that I did for the last seven-plus years. It really does start wearing on you. Now I don’t have to worry about any of it. It’s all out there.”

Muhl often shares information about her condition, and even shows off her stomach on Instagram. She revealed her diagnosis in a post last year, boldly writing, “I’m a fraternal twin who fused together with my sibling in the womb.”

She tells PEOPLE: “I don’t have to hide who I am anymore and it’s a great feeling.”