When I lie down and then get up to walk, I experience a heavy feeling or difficulty in breathing for about 2 to 3 minutes. I also notice a heaviness in my upper abdomen during this time. After those few minutes, I feel normal again. I've had an X-ray, CT scan, ECG, and a 2D echo, and all the results came back normal. Should I be worried about this, or am I just overthinking it?