I think you may be doing a valsalva maneuver without realizing. This is common. Maybe your PCS has just aggravated the problem.

http://www.ems1.com/ems-products/edu...alva-Maneuver/

The person will inhale, close the glottis, and tighten the chest and abdominal muscles. This increases not only the intra-abdominal pressure to assist with colon evacuation, but it also increases intrathoracic pressure and produces the Valsalva effect.

The end result may cause the patient to experience a syncopal episode due to a decrease in cerebral blood flow and cerebral perfusion, especially if they have cardiovascular disease. While the Valsalva effect may occur during bowel movement, it may also be experienced if the breath is held while lifting heavy weights or while straining when moving heavy objects.

Also, do you take tylenol / narocotics or any meds that cause constipation? You can take a stool softner/ colace or try senna tea.
I used this when I was taking narcotics for the pain.http://www.traditionalmedicinals.com/smoothmove/
Happy to hear of your lack of headaches.