Hi Amber, and thanks for your post and question - it's a good one! A flare-up, sometimes referred to as an exacerbation, is a sudden worsening of one's symptoms. They can occur without warning or, build up over several days/weeks until they become very apparent for the patient. They can vary considerably from patient to patient as well.
I thought you might find it helpful to look over this article on that very topic: https://copd.net/living/first-exacerbation. I do hope you find this information is helpful in a practical way. We actually have many more articles which focus on this important aspect of a COPD diagnosis. Should you be interested in reading more, please let me or any other moderator / team member know. We will be happy to point you in the right direction.
Wishing you well,
Leon (site moderator COPD.net)