Grats to you for having a well-functioning and loved 27 year old!
My first question would be: Have you had her teeth checked as of late? Most likely not the problem because of her age, but never hurts to check for sharp points, abrasions, etc.
Next I would ask if she has had any chiropractic care in the last year or two? Could be a recent adjustment has caused some stiffness. Even more likely, if she has not had an adjustment, her new bit may be causing her to carry herself a bit differently, causing stiffness in muscles she has not used before (or in a long time).
Lastly, has she gained or lost any weight? Your saddle fit could be off, causing tenderness that may go away with muscles warming up, and when asking for a deep flex/bend, could be pushed back into a painful state.
Best of luck, and be sure to keep us updated!