Ms Denton, whose baby is due in December, shared a screenshot of the exchange on Facebook earlier this month asking if the manager's reason for sacking her was "allowed".
Her post has since been shared and liked thousands of times and the young woman has been inundated with comments and advice.
"That's absolutely illegal. Sue the company and that person individually," one Facebook user recommended.
Another wrote: "Pretty sure you can sue! They legally cannot do this, you need a job to support your family an for them to fire you because you're pregnant they will get into huge trouble."
"A workplace is also supposed to give you three months of maternity leave, unless you choose to take a shorter time … them stating that you are not allowed to work at a place due to pregnancy is absolutely ridiculous," another person added.
Ms Denton responded, confirming she was "definitely going to sue" the franchise.
In an interview with TV station KIRO7, Ms Denton said she was stunned after reading the message.
"I was just like in shock, it took me a minute to face reality I was like this is really happening," she said.
The station then reached out to business owner Tim Trieb, who said the manager who fired Ms Denton had resigned.
"(It) should have never ever happened … It's our policy to treat everybody equally," he said.
Mr Trieb said he had offered to give Ms Denton her job back, but she had refused.
She said she had received a number of other job opportunities since her story went viral.
