Period poos suck. If there's a God out there, why do we even get period pains and period poos? 

Please tell me. What causes them??

I could be around the bush about this one but the answer is hormones. (Sigh.) The answer is hormones.

"During menstruation, chemicals called prostaglandins are released, which stimulate the womb to contract and shed its inner lining," - Dr Karan Raj.

More prostaglandins dingling around in your body the more pain you feel. Simple.
If you need to hear a doctor go on about it, I include a longwinded quote below.

"Higher level of prostaglandins equals more cramping, [and] when the wall of the womb contracts it also compresses blood vessels lining the womb. This temporarily cuts off the blood and oxygen supply to the uterine tissues and without oxygen these tissues enter a state known as ischemia. Ischemia, causes the release of chemicals which cause pain and the release of more prostaglandins, which makes the womb contract even more." 

Seems like a pretty annoying cycle of pain on pain.

These pains make your poo guts contract too, resulting in diarrhoea...period poo.

If you're having pains or period poo;s rest assured it's a normal part of menstration.

The professional advice is this: If period pain is preventing you from continuing normal activities,  seek medical advice.