When dogs pant a lot without a simple reason such exercise, heat, or sexual interest, they are usually experiencing stress of some kind. If your household has changed in any way or if your daily habits have changed, she might be responding to that. I don't know your dog or your habits so I can't really say.
Have you had more or different people over? Are you doing summer things that don't include her? Did you add a pet? Add a relative? Have you changed your schedule at all? Is Tristan gone more than usual?
She's probably fine but upset.