It's not that dreadful Nas but what your describing is what I suffer,a course of ABs should shift it,the only way it can be diagnosed is with a colonography or ct colonoscopy,I'd push for the latter tbh
IF and it's a big if,it is diverticulitis,it's very common as you age,it's little pouches in your colon that become infected,I always keep away fro seeds,pumpkin,poppy etc as these are the things that can get trapped.
Mine is very sore when I press on it and also if I need to poo, I can feel it worse then too.