Lower back pain, constipation, gas, frequent urination, and all tests were negative for infection. What could this be?

Uncertain. It could be the constipation, but there can be other issues that can cause constipation and some of the symptoms. This could be a muscular problem, pulled muscle, tendon, or even a bursitis. You need an evaluation to help clarify your symptoms.
Needs investigation. Complex symptoms such as these need a full history and investigations such as a colonoscopy or MRI to rule out sinister causes such as malignancy. It is prudent that you see a specialist if your family doctor is unable to determine the cause of your symptoms.

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