Why would a well blended drink of fresh pear, parsley, celery and plain yogurt cause intestinal and back pain?

Biliary Colic?: You may be describing "biliary colic." this is mid epigastric (pit of stomach) discomfort that can extend through to the back when something stimulates the gallbladder and bile ducts to contract. This can occur in response to the intake of certain foods. However it may also signal inflammation of the gallbladder and the presence of gallstones. Consult with your primary care or GI doctor.

Lactose: You could have lactose intolerance or other food sensitivities.

Fruit juice and milk: Fruit juices and milk products (yogurt) can often cause bloating, gas pains, diarrhea and should be taken with caution if you have a history of easy stomach upset, irritable bowel syndrome, lactose intolerance or other related conditions. It is unclear why this would cause back pain unless you are having muscular pain from the cramps.