I am sorry to hear you are worried about Sophie. How is she doing now?
My name is Rebecca. I am a veterinarian and will do my best to help.
There are different things that can cause a dog to suddenly pant, and be uncomfortable. Pain, fever, heart or lung problem, blood clots. If I could, the first thing I would check is the color of her gums to be sure they are pink and not pale. I would feel her pulses, and take her temperature. I would listen to her lungs and heart with a stethoscope. If that all seemed normal, I would try to palpate her neck, back and legs trying to find an area that is painful. Sometimes this is just an area where muscles are a little more tense than normal. When dogs show pain, they often do not moan or cry or whimper; they pant and act restless.
Pain can also be internal: something she ate, pancreatitis, gas pains.
Since she is only 6, a heart problem or blood clot is less likely.
How is she doing now? Let me know what other questions I can answer.