I'm 66 Years Old. I Don't Have A Weight Problem.

I'm 66 years old. I don't have a weight problem. I eat healthy most of the time. I'm a group exercise instructor and currently teach chair exercise to older adults and also work out at a gym at least 2-3 times/week. Lately, the last couple of days I have experienced pain on my back toward the right side, right above my waist. I also worked very hard raking leaves last week for a few hours. I wonder what is causing the pain (off and on)- no contant? Thanks!