I'm a biologist but I can't figure this out

[Biologyman]

Blood (dark red, not bright red or black) started appearing in my stools about 2 months ago and increased in frequency to a daily event. There was no associated pain although my stools became looser. I had already had a routine colonoscopy 9 months ago (a UK NHS screening present when you reach 55) and the rectum, descending and transverse colon were as clean as a whistle. My concerned GP ordered another colonoscopy 3 weeks ago and this showed some areas of bleeding/inflammation in the rectosigmoid section up to 30cm. I looked carefully at the colonoscopy screen myself and there were few areas of erythema (and certainly no polyps). 4 tissue biopsies were taken that later showed no evidence of carcinoma (phew!). I then took a 10 metrodnidazole course (specific for anaerobic bacteria – but there again, most colon bugs are anaerobic. Goodbye healthy gut bacteria) that had absolutely no effect.

So, I’m stumped. It’s not cancerous, it's not the result of a bacterial infection and it doesn’t cause any pain. So what’s causing the inflammation? It’s possibly a coincidence but immediately prior to the bleeding starting, I’d been travelling around Albania and eating in local restaurants. Lovely people, delicious food but not an EU country and so possibly not up to speed with hygiene rules.

Has anyone else experienced similar symptoms and/or knows a diagnosis? Intellectually, it’s really niggling at me. Many thanks, everyone.