What symptoms do you have, besides the back ache? Do you have pains anywhere else? What kind of pains: aches, cramps, stabbing? I assume you are having diarrhea? What's the consistency and shape and color of your stools? Do they burn when they come out? Do you have any nausea and/or vomiting?
Without more information, I'd hazard a guess at maybe a bad gall bladder, or maybe a "female" problem. Pregnancies make gall bladders go bad frequently for some reason. Also, gynocological problems like messed-up hormones, fybroid tumors, poly-cystic ovary syndrome, endometreosis, etc. can cause both the lower back pain and bowel problems. Sometimes having children can trigger those things (certainly did when my mother had me). Also, if you had a C-section, sometimes scar tissue from the incision can mess up your intestines. I think someone else on here had exploratory surgery and found that scar tissue from some gynocological surgery(ies) had entangled her intestines. My mother developed a fybroid tumor which managed to attach itself to her intestines and caused severe pain and bowel problems. The very bizarre thing was that this developed AFTER she'd had her uterus removed. Her doctor said he was going to submit a paper to an academic journal on it, it was so weird. They think the tumor formed on a piece of endometreosis that had gotten detached from the uterus when they removed it. So, all sorts of weird things can happen down there.