I am not sure specifically why an ECG was organised but it maybe that the doctors picked up something on a chest x-ray/ or listening to your heart that needed further investigations. As the heart is close to the lungs they may have wanted to check how your heart was working. As I am not involved in your care it is difficult for me to be too specific.
You can have a look at the NHS information on having an ECG here
I can understand waiting for investigations to happen and then waiting for results can be unsettling but it is important that your doctors get as much information as they can in order to get a complete picture.
When you next see the doctor don't be afraid to answer questions so you have more of an idea of what they are investigating.
Hopefully they will be able to tell you more when they get the results.
I do hope that you know more about your situation soon.
