welcome to healthcaremagic
from your description of symptoms you might be having vasovagal syncope
which makes you feel all of sudden tired have dizziness
and feel faint. it is usually preceeded after bearing down or straining alot.
other cause could be nervous problem may be you could be suffering from abscence seizures which leads to fatigue
in post ictal phase.
i would suggest you need to be done investigations to rule out what could be causing the symptoms mentioned above,
i would recommend electrocardiogram to be done with ct scan
of the brain with electroencephalogram to be done.
would suggest you visit a specialist to do full work up and further treatment
hope have been able to answer your query
get well soon takecare.