FENTON, Mo. (CBS Local) — A concerned mother says she took her baby out of a suburban St. Louis daycare after she saw him drink another woman’s breast milk.

Jessi Werkmeister said she was shocked to see her child being fed with a bottle that wasn’t his when she picked him up at Wee Care Childcare in Fenton early this month.

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“I was really mad at first, I was just so mad,” she told CBS affiliate KMOV.

Werkmeister said her son was born four weeks early and is having “tummy issues,” so she was sending him with a special formula. But she said she soon found he was drinking milk another mother had pumped.

“He started spitting up about a half hour after we got home,” Werkmeister said.

Werkmeister said her son has a distinctive birth mark that should have alerted the staff.

“It’s a horrible feeling knowing your child is getting breast milk from another mother who you don’t know their daily habits, if they eat well if they smoke or drink,” she said.

Werkmeister said she tracked down the other mother on Facebook.

“She reassured me it was breast milk,” Werkmeister told KTVI. “She said, ‘What is my son getting if your son’s getting my breast milk?'”

Wee Care Childcare apologized to “the parents of all parties involved in this isolated incident” and said “this issue has been addressed and resolved” in a written statement.

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But Werkmeister said her baby was not only drinking from the wrong bottle but was also wearing a diaper “labeled with the other child’s initial and it was not the brand that I buy.”