By Lisa Respers France, CNN

(CNN) -- Alyssa Milano says she has tested positive for coronavirus antibodies after having tested negative for Covid-19.

The actress and activist is now taking aim at what she says is a flawed testing system.

On Wednesday Milano posted a photo of herself using a breathing apparatus on her verified Instagram account and wrote in the caption, "This was me on April 2nd after being sick for 2 weeks."

"I had never been this kind of sick. Everything hurt. Loss of smell," she wrote. "It felt like an elephant was sitting on my chest. I couldn't breathe. I couldn't keep food in me. I lost 9 pounds in 2 weeks."

She said she felt confused, had a low grade fever and a terrible headache.

"I basically had every Covid symptom," Milano wrote. "At the very end of march I took two covid19 tests and both were negative. I also took a covid antibody test (the finger prick test) after I was feeling a bit better. NEGATIVE. "

What followed, she wrote, was four months of lingering symptoms including vertigo, heart palpitations, shortness of breath and short-term memory loss. Milano went and had her blood drawn for another antibody test.

"I am POSITIVE for covid antibodies," she wrote. "I had Covid19. I just want you to be aware that our testing system is flawed and we don't know the real numbers."

"I also want you to know, this illness is not a hoax," the "Charmed" star added. "I thought I was dying. It felt like I was dying."

Milano, who also shared an image of her positive antibody test result, said she plans on donating her plasma in the hopes of helping others.

She also had words of advice for her followers.

"Please wash your hands and wear a mask and social distance," Milano wrote. "I don't want anyone to feel the way I felt."