I'm sorry to hear of this with Trixie. Panting and shaking can represent both medical and behavioral disorders.
For example, excessive panting is often seen with elevated ambient temperature, exercise, anxiety, or pain. To confound the diagnosis further, it's also seen with fever, narcotic administration, glucocorticosteroid (prednisone, e.g.) therapy, Cushing's disease, hyperthyroidism, hypocalcemia, pheocromocytoma (an adrenal gland tumor), cardiac disease, tachyarrhythmias (fast irregular heart rates), brain disease and obesity.
Let's "watchful wait" and see if a track record of Trixie's behavior is established. A one-off panting and shaking could indicate something as simple as a cramp. On the other hand, such behavior concomitant with difficulty jumping up or down, navigating steps, and vocalizing when approached or lifted could indicate spinal pain. We need to see what or if something more develops. Please respond with further questions or concerns if you wish.