Personal trainer Vicky Veness, 30, had been experiencing a recurring cough but doctors had originally brushed off her symptoms as asthma.
The exercise fan, from Cheltenham, posted a picture of herself looking fit and healthy on Facebook, with a comment saying: 'Just a warning that this post may be upsetting to read.
'This photo was taken a few hours before I was diagnosed with stage 4 lung cancer. I'm 30 years old, a personal trainer, runner, non smoker and healthy eater.
'When you have cancer you won't necessarily look ill on the outside. The symptoms might be much more subtle and only show themselves occasionally.'
She went on to say: 'Unfortunately my symptoms were brushed off as asthma by many general practitioners.
'The moral of the story is this, if you feel unwell for whatever reason, it doesn't matter how silly you might think it might be, see your doctor, question everything and keep going back until you get the answers you need.
'This has been the most emotionally and physically challenging week of my life, but now it's time to fight this!' #nowletsfightthis #roadtoremission #cancerawareness #standuptocancer #cancerresearchuk #macmillian'
The post, which has since had 600 shares on Facebook, was posted by the PT in order to raise awareness. Talking to MailOnline, Vicky said: 'I decided it was important to share my post in order to highlight that cancer really can affect anybody, in any walk of life.
'When most people think of lung cancer they immediately imagine somebody who smokes. I want to make people aware this is not always the case.
'It worries me that 'healthy looking' people and non-smokers may be overlooked when diagnosing this condition. It took 18 months to reach my diagnosis.'