What Causes Mild Pain In Lower Left Back And Frequent Urination?

Question: For the past couple of weeks I've had a low grade pain in my lower left back (shank area or just above the iliac bone). I'm a 50 year old male and my last digital rectal exam was around 5 years ago. My father was diagnosed with prostate cancer at 50, had surgery, and is still alive today at 78 with a low PSA. I noticed recently that I've had to urinate frequently in the morning on some mornings (although I do have several drinks the night before fairly frequently). Now I'm terrified that these symptoms indicate that I have prostate cancer that has metastasized to the bone. I have an appointment with a new GP on Tuesday but am very frightened today having pieced together the lower back pain with frequent urination. Other factors: I have no pain during uriniation or blood in my urine, although on a couple of occasions recently I found it difficult to start urine flow, and it was a week flow. Current meds I take are Prozac and Wellbutrin. I also saw a doctor for the back pain this past week and he gave me a urine test to check my kidney, and it was clear. Does this sound quite certain that I have prostate cancer and that it's spread?

Thank you for that information. I will follow up for the digital, sonogram and PSA tests that you suggested.

As I am very worried, I bought rubber cloves and lubricant from my local pharmacy and just attempted to give myself a digital rectal exam based on instructions on the internet (with finger nail facing up towards my belly button). Prostate tissue seemed soft and there were no big lumps or hardness that I could detect, although I did seem to feel something hard that seemed very small, perhaps smaller than a sesame seed. I don't know if this is some kind of scar or if it's the sulcus of the prostate, or if it indicates prostate cancer. And if so, would something that small be able to have spread to the bone?

Thank you.