Mallory Smothers uploaded a photo of two pouches of breast milk - one pumped before her baby became ill and one after.
The mother believes, after reading an article from a medical journal, that her milk had produced antibodies to help her baby fight off the illness.
"This doctor discusses that when a baby nurses, it creates a vacuum in which the infant's saliva sneaks into the mother's nipple," Smothers explained about the article she read.
"There, it is believed that mammary gland receptors interpret the 'baby spit backwash' for bacteria and viruses and, if they detect something amiss, mum's body will actually change the milk's composition by producing customised antibodies."
Smothers also cited a 2013 Clinical and Translational Immunology study that found when a baby is ill, the numbers of leukocytes (cells in the blood) in its mother's breast milk spike.
"I filed [the information] away in the back of my mind until I was packing frozen milk into the big deep freeze today," Smothers continued on the post.
"I pumped the milk on the left Thursday night before we laid down for bed. I nurse baby every two hours or so overnight and don't pump until we get up for the day.
"I noticed in the wee hours of Friday morning, she was congested, irritable, and sneezing a lot. Probably a cold, right?
"When we got up Friday morning, I pumped, just as we always do. What I pumped is on the right side of the photo."
Smothers believes the milk on the right resembles "colostrum" - the "super milk full of antibodies and leukocytes" produced during the first few days after birth.
The Facebook post, uploaded on Sunday 14 February, has been shared more than 70,000 times in nine days.
Other mums claim their breast milk also changed colour when they were feeding their sick babies.
"I breastfed both my two and found this to be very true - our bodies are truly amazing," wrote one mother on the post.
"I've actually noticed that before too," another commented. "Just didn't realise that's why my milk was different."
In her post Smothers made reference to research by Katie Hinde, PhD, a biologist and associate professor at the Center for Evolution and Medicine at the School of Human Evolution & Social Change at Arizona State University.
In October 2015, Hinde told journalist Leslie Goldman in an email interview that when a baby nurses, it creates a vacuum in which the infant's saliva sneaks into the mother's nipple.
Hinde said, according to Mom.me: "There, it is believed that mammary gland receptors interpret the 'baby spit backwash' for bacteria and viruses and, if they detect something amiss (i.e. the baby is sick or fighting off an infection), her body will actually change the milk's immunological composition.
"Putting this all together, some scientists hypothesise that this could be one of the ways babies let mums 'know' about their condition and mums respond with infection-fighting antibodies."
Janet Fyle, professional policy advisor at the Royal College of Midwives said it is certainly possible the mother's belief that the composition of her milk changed is correct.
"The body does a lot of stuff we don't understand," she told The Huffington Post UK. "The mother might be right and breast milk does change sometimes.
"The mother has quoted what a scientist has said - who tend to know a bit more than the rest of of us. I don't want to dispute what she's saying.
"The body does wonderful things and so does breast milk. The saliva reaction she talks about is when the nipple absorbs bacteria and realises the baby is unwell.
"So it is possible - never underestimate the power of the body."