This is probably due to the fecal reflex. What happens is that when we pass the stools, the vagus nerve may get stimulated and this causes the blood pressure to come down. In most situations, the body compensates for this and one doesnt feel symptoms.

In your case, straining at stools may be the cause of the vagus stimulation, leading to fall in bp , and thus the symptoms, including the passing out.

Take plenty of fluids regularly.
Check the follows- FBS, PPBS, HbA1C, lipid profile, TSH and ecg

Let us know the reports so we can help you better
Best of luck
Dr Mittal

Hi Welcoe to HCM
Your situation is matter of concern . But nothing to worry ,it can be rectified by making few changes in your life style .

Your problem is directly related to your faulty life style , faulty food habits and thus spoilt digetion systen .

Medications such as pain killers and anti-depressants, spinal injuries and muscle disorders, inadequate exercise, advanced age, and pregnancy & lack of dail can all lead to such situation .

Increase the amount of fiber in your diet over a period of weeks. and intake of lot of water in your Foods that contain fiber are whole grains, fruits, vegetables and beans . Include proper amount of protien vitamins and minerals in natural form

Take lemon juice in water a glass or two .

Make walk , exercise , yoga / pranayam an integral part of your regimen

Avoid Fried ,fast foods , bakery products , Tea , coffee , alcohol . soda

Meawhile take , Aloe vera ,cleans and heals the digestive tract and helps soften stools. Drink 1/2 cup of aloe vera juice twice daily.

Garlic helps destroy harmful bacteria in the colon which can cause constipation .
Coconut water also helps in maintaining goog health .a very good antioxidant
Start you above regimen and for get bad day . you suffered

If your signs and symptoms remain the same or worse, tell your doctor.

Hope this helps in solving your query . All the best Recover soon
Do get back if have any further query