If all her investigations are normal, it is highly likely that she did indeed have a panic attack.
For termination of panic attack we can give SOS medicine. But simultaneously you need to find out if there's any stressor that is precipitating that attack. Once the stressor is identified, we can focus on tackling that. Once that gets resolved, the attacks should subside. Till then start her on medication .
Also , she can start some form of exercise and practise relaxation techniques.
Next Steps
consult a psychiatrist at the earliest