Doctors are seeing several women coming in for mammograms with the same issue and it's causing concern.
The patients have swollen lymph nodes, which is a rare sign of breast cancer.
"When we see that, we perk up and get worried," said Dr. Lars Grimm of Duke Health.
Doctors are now learning this is caused by women who recently received the COVID-19 vaccine.
Lymph nodes will be swollen on the side of the body the person received the shot.
"That's a good thing, right, because that means that the vaccine's working. Your body is responding. You're having that immune reaction," said Grimm.
The side effect, though, is that those lumps will show up a mammogram and give a false reading for breast cancer.
Grimm says women just need to do some planning.
Either get the mammogram screening done before the first COVID-19 vaccine dose, or four to six weeks after the second shot.
Grimm says the recommended timetable is the same whether you're getting the Moderna or Pzifer vaccine.
The last thing physicians want is women canceling their mammograms all together, which happened often in 2020.
"We're concerned women are going to skip two years and we know that when we screen regularly, we catch breast cancer at the earliest stage possible. Unfortunately, breast cancer is just so common, it's really important that women stay on top of these health maintenance things and don't let it slip," said Grimm.
The American Cancer Society suggests women 45 to 54 get the screening annually. Women 55 and older, in good health, can switch to every other year.