To me the breath also usually becomes the central point of awareness after a gentle body awareness. That's because the breath is the most prominent sensation in the body, so that's where my attention is drawn to. I am not sure what the excitement feeling was, but I wouldn't give it importance. Just keep with the breath until it begins to be very fine and peaceful and it is the only thing you are aware of.
The sad/low feeling I would say had no direct connection with the meditation.
From a fellow practitioner,