When I lie flat I can't breath good. Just feel short of breath . Then I get up and still feel like that for 10min or so then feel ok. No problems during normal activities. Just lieing flat.
What you are describing is called orthopnea, which is shortness of breath or difficulty breathing comfortably when you lie flat.
It can have many causes: asthma, chronic bronchitis, sleep apnea, even panic disorders. Another cause could be certain heart related issues. Also GERD (gastrointestinal reflux disease, i.e chronic heartburn).
May I ask how old you are?
And how is your health otherwise?