I'd consider more aspects.

First of all - just because she hasn't worn tennis shoes on your session days for 2.5 years doesn't mean that if she now does, it HAS to be health-related.

I would think it's okay to ask if you're worried, after all you've been seeing her for some time now and she's the one who mentioned health issues in the first place. She may or may not want to get into the specifics, but she did open a door there.

I wouldn't go telling a female therapist she's gained a lot of weight. Maybe that's just me. Actually, I wouldn't say that even to my male therapist if something like this happened. Personally I'd phrase it something like "You've mentioned the possibility of retiring due to health problems and I'm worried about you. Is everything alright?" This would work for me and my therapeutic relationship, at least.