7 day heart monitor

Im having a 7 day heart monitor next week, i had open heart surgery when i was 4 to repair a hole in the heart, had no problems since operation even during pregnancy i had no problems. Since back end off last year been suffering from breathlessness and fast heart beat at random unexpected times during the day even when im just sat down doing nothin. im a bit nervous now its close to happeining because if they say they find nothing im going to feel like a prat, also nervous about how uncomfortable and how noticible it will be. Has anyone had a 7 day monitor? how did you find it?