I would reply something like, "Thanks, Mom, and I love you too!"

I would not take it personally since it doesn't sound like you have a problem. It sounds like she has a problem with your getting older and taking care of yourself, etc. She can't be as "in charge", can only manufacturer ways in her mind that you might need her. I'm with CindyLu, tell her you'd be glad to go to the gym with her to help her with her problems.