Why do i get dizziness and weak the moment i want to go toilet?

Depends: As with all patient complaints there are many possibilities. If you become symptomatic when you strain you should go get evaluated

Vagus Nerve: When we strain our parasympathetic nervous system, which is the balance of sympathetic nervous system-Adrenaline, gets stimulated and a large nerve called the Vagus nerve can tell the heart to slow down and blood pressure to go down. I suggest staying as regular as possible with prune juice, water, fiber so u'r colon won't have to work as hard.