unless she is wearing an MS tshirt I would not do it. It may be a stroke she had and she may be recovering and your innocent comment may make it clear that others really see her disability.

What you may do is slow down your bike to walk it as you get near her. As you are passing say something like "whooo I have MS and this heat is really pushing me. ...how are you doing today?" and let her figure out if she wishes to disclose what she has. This way the only person you are outting is YOU. If she wishes to remain hidden she can. She may turn out to be chatty and really need to share, or she may be embarrassed and just want to hide.

Im so glad you want to reach out to her. You are a love.