Eighteen-year-old Sara Thompson studied hard in the months before her final exams, but the amount of time she spent in a sitting position nearly killed her, London’s Daily Mail reported.

Thompson developed deep vein thrombosis, a blood clot in her leg vein, and her right leg was so swollen by the time she took her English literature exam, she couldn’t even walk.

Doctors told her if she waited one day longer, she might have lost her leg or died.

“I thought I had pulled a muscle,” Thompson said. “But over the period of a week, it got worse. After sitting my English exam, I was in so much pain. I (called my doctor) who told me to go straight to the hospital. It was there that they told me I had DVT.”

Before Thompson developed the blood clot, she spent many weeks at home, sitting and studying. She wasn’t moving around much, which is how many blood clots can occur. Doctors also believe that taking the birth control pill contributes to blood clots in young women.

Thompson, who spent five days in the hospital, is still battling the effects of the blood clot, and she was unable to finish her exams.

“My advice to other (students) is to make sure they get up and walk around every hour or two,” said Dr. John Scurr, a consultant surgeon at the Lister Hospital in London, who is also an expert on deep vein thrombosis.

Click here to read more on this story from the Daily Mail.