Could my constipation, lower back pain be symptoms of bladder cancer?

Not usually. Bowel symptoms and back pain would be unusual bladder cancer related symptoms. Blood in urine and/or painful voiding would be much more common.
Maybe, but unlikely. Constipation and low back pain should be evaluated by a health care professional. Blood in the urine (either seen or found on a urinalysis) is usually the first sign of bladder cancer.