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This complaint almost certainly means you have a prostate issue.
These break down into three varieties...
Acute prostatitis, an infection of the prostate which comes on rather quickly and causes decreased urine and, usually, a feeling like you have the flu... low back pain is often present.
Benign prostatic hypertrophy. An enlargement of the prostate that, over time, decreases uring flow and makes it hard to empty your bladder fully.
Prostate cancer. the same symptoms as Benign prostatic hypertrophy, but this IS cancer.
The point is that at this point you absolutely need to see your doctor for evaluation.
This will include a blood test, a urine test, and will likely require a referral to a urologist, a surgeon who specializes in diseases of the urinary tract and prostate.
THIS LINK HERE IS A VERY GOOD PLACE TO LEARN ABOUT THESE POSSIBILITIES.