Stress related. In addition to high variability in cycle duration and flow amount in otherwise normal women, there is relationship between stress and menstrual experience. In that context, anything that causes psychological distress can influence menstrual flow. Panic attacks is one of the most distressing experiences for human beings. Luckily, it is now quite treatable by modern medicine.
Panic & bleeding. Unless you have menstrual cycle difficulties or taking medications that attribute to changes in flow, not likely.