Re: sharp pain in left side? help please!
Your pain could be many different things. Could be musculoskeletal(some sort of atypical strain) vs. atypical mononeuropathy(some sort of subcostal nerve compression/entrapment) vs. radiculopathy(spinal nerve root compression). Could very well be neurogenic as you describe it responding to sitting down or lying down("classical description for neurogenic claudication although your described location is definitely not classical). If it continues, you'd benefit from seeing your PCP to start the work up.
Would be weird for diverticulitis picture of pain, esp lasting on order of weeks +, expect you to be sicker than what it sounds like you are.
Alternatively, if you've ever had shingles in that distribution could be atypical post herpetic neuralgia.
In the end, you'd benefit from seeing your doc.
best of luck