I would suggest you get this checked out by your primary care provider. Lower back pain in at the time of a bowel movement can represent two unrelated issues (for example, maybe the back pain is caused by straining due to constipation), but it can also be a sign of other conditions, including prostate problems or endometriosis (I don't know if you're a man or woman, so I'm covering both bases).
Also, start taking a good look at your stools (bowel movements) and note what they look like. In other words, are they soft and brown? Black? Oily? Loose? Your health care provider will want this information to help diagnose the problem quicker.
I hope you're able to get this figured out soon, since it sounds like it's having a negative impact on your everyday life!
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