I started having severe pain in my lower back and left side abdomen just under my bottom rib, should my doctor do other tests?

I am a 69 year old male. 19 days ago today just out of the nowhere I started having severe pain in my lower back and left side abdomen just under my bottom rib. That pain lasted all day but the following day was just like it never happened. Then the third day was back to the same pain again then the fourth day nothing. This pain cycle repeated several times then after 8 or 9 days I started noticing a black tarry stool. There was never any fever and no constipation and only very mild softening of stool during the whole time.The pain episodes were diminishing and the tarry stool was looking normal in about 2 weeks but then I started noticing a lot of mucous surrounding my stool. I had been busy on a work project this whole time and had put off a doctor visit until the 16th day. I called my doctor’s office and went in that day. The doctor did urinalysis, stool test for blood and did bloodwork to check for infection, etc. All these tests came back normal so he scheduled a CT scan for the following morning and the ct scan also came back normal. The doctor put me on ciprofloxan 500mg twice a day and metronidazole 500mg 3 times a day. I had a colonoscopy 2 years ago and 3 polyps were removed.
This obviously was/is a GI issue but how could all this have taken place and yet leave no traceable evidence? Should my doctor do other tests or should I just consider this like a health hiccup and forget it?