Cameron McMillin suffers from Ehlers-Danlos syndrome (EDS), a connective tissue condition that causes pain in her joints and muscles.
After she was spotted leaving her car at a school in California, someone couldn't help but notice that she looked "able bodied" and therefore wrote her a snarky note.
McMillin has since posted a response on Facebook, where she explains why she has every right to park in the disabled bay - and to educate others not to judge so quickly.
The note that was left on her car said: "Ladies, it appears that you are both able bodied. It’s disappointing to see the misuse of that disabled placard here at Rocklin Elementary.
"Please reserve this spot for people who are truly less fortunate than you. Thank you."
After reading the letter, Cameron McMillin - one of the so-called "able bodied" ladies - wrote a passionate response on her Facebook page.
She said: "I think it's my turn to say something. Hello. My name is Cameron McMillin. Today you made me cry, but that's okay. I'd like to take this time to inform you about a medical condition that I have called Ehlers-Danlos Syndrome.
"It is a connective tissue condition that causes pain in my joints, muscles, and all throughout my body, dislocated hips, knees, ankles, and ribs, heart palpitations, organ problems, multiple surgeries, and many more.
"You have no idea what I go through every single day. I know you made a snap judgement by what you saw, but what you see on the outside is not what exists on the inside.
"My body may be weak, but I am strong. Next time please get to know a person before you make a comment like this."
She added: "People who have Ehlers-Danlos Syndrome are never heard and I want to change that. It's about time we gain a voice."
According to the NHS, Ehlers-Danlos syndrome (EDS) is a collection of inherited conditions that fit into a larger group, known as heritable disorders of connective tissue.
They can cause fatigue are people who suffer from the condition are required to conserve their energy levels.
"It is recommended that those with vascular EDS wear a medical alert bracelet," the site reads.
Since she posted the note on Facebook, commenters have been quick to share their experiences of being discriminated against for their invisible illnesses.
Pamela Weaver Whitted wrote: "I'm sorry it made you upset. It upsets me. I also have EDS. This sort of things happens often and it's so wrong."
While Stephanie Baker added: "I have to tell you I get dirty looks all the time when I park there on my really bad days. EDS is tough and people have no idea. Thank you for being strong and saying what needs to be said."
Also on HuffPost: