Maria Carreiro, 51, said she "ran down the street like a crazy woman" after her daughter looked online and discovered the award's true size.
Among her plans for the winnings were a Hawaii honeymoon after 30 years of marriage and a larger house.
Ms Carreiro had worked at a factory before quitting to raise her children.
She said her husband had already left his job.
"I couldn't believe it," Ms Carreiro told reporters. "I was shaking and just couldn't believe that it was real."
Ms Carreiro bought the LottoMax ticket for a Friday drawing and checked her ticket at the store on Saturday morning. She realised her multi-million dollar mistake when she returned home to tell her family.
According to Ontario Lottery and Gaming, 60% of winners of more than C$1m paid off debts, 56% took a vacation, 36% bought a new house and 43% either retired, changed jobs or quit.
A Toronto woman celebrating what she thought was a 40,000 Canadian dollar ($39,428; £26,000) lottery prize had in fact won C$40m, officials have said.