Shauna Lawson, who has four tumours in her left breast, was told by doctors her mastectomy may or may not go-ahead because of the strain on the health service.
Ms Lawson is due to undergo surgery on Tuesday morning at Belfast City Hospital, but that could change.
She described the situation with her treatment as “surreal”
The mother-of-two said she cannot believe the situation she finds herself in and people’s actions during the pandemic would have “a knock-on effect on life-saving treatments”.
“We are running out of ventilators and ICU beds and I don’t think people realise just how critical that will be if something happens to them or someone in their family,” the 35-year-old added.
She said she hopes others will take the new lockdown measures seriously and to think of how their actions could affect people in a similar position to her.
“People really need to open their minds, have a look at themselves,” Ms Lawson said.
“Please stay inside”
A spokesperson for the Belfast Health Trust said the “situation was changing rapidly because of Covid-19, but people would be notified individually and people would be contacted individually if their surgery is postponed”.