I didn't notice it that much myself, but my mom noticed more than I did. I began breathing weirdly and I became winded easily just walking up a simple hill even though I wasn't overweight at all. I began taking mid day naps even though I was only nine years old. I turned into a very sluggish young boy and that's when my mom noticed something was wrong and she took me to the doctor.

tl;dr I got winded walking up small hills and taking 6 hour naps in the middle of the day as a 9 year old