It could be vasovagal syndrome effecting parasympathetic nervous system (vagal tone) and that will effect hemodynamic (blood pressure and heart rate) responses.
This results in a spectrum of hemodynamic responses:
On one end of the spectrum is the cardioinhibitory response, characterized by a drop in heart rate (negative chronotropic effect) and in contractility (negative inotropic effect) leading to a decrease in cardiac output that is significant enough to result in a loss of consciousness or lighheadedness. It is thought that this response results primarily from enhancement in parasympathetic tone.
On the other end of the spectrum is the vasodepressor response, caused by a drop in blood pressure without much change in heart rate. This phenomenon occurs due to vasodilation, probably as a result of withdrawal of sympathetic nervous system tone.
The majority of people with vasovagal syncope have a mixed response somewhere between these two ends of the spectrum.