I will be the contrarian here, having been a vet for a buncha' years ... and our patients can't tell us what's wrong ...
SO we have to go looking AND y'all know how I feel about LOOKING !
" You will miss more for not LOOKING ... than not knowing. ! "
Your Doc is correct to go top down in the initial diagnostics : get as much info at the start and THEN refine the scope of diagnostics as the results fit the patient's presentation.
I know we are a CRC forum and we all want to encourage everyone to have c-scopes .... BUTT as both a clinician and Stage IV CRC survivor myself, IF I were in your place with your presentation right now, I would get the contrast abdominal CT first AND the c-scope as indicated later.
The biggest mistake docs and patients make is assuming the diagnosis and getting a test to confirm or prove it.
The best diagnosticians I know always get the big picture first and refine the diagnosis along the way... getting to the correct diagnosis because they have not left anything out along the way.
At last count I'd had 18 full C-A-P CTs with contrast = BTDT and I still don't glow in the dark
Best wishes whatever you decide