Constipation followed by diarrhoea with dark coloured blood and mucous.. is this a sign of crohns? fatigue & in lots of pain around naval

Possible: it is always possible based on those symptoms. however it could also be an infection. you should seek gastroenterologist evaluation and possibly a colonoscopy, particularly if symptoms have been present for over a month

Dark bloody stool: You should go to the ER today( preferably now) . You can lose a lot of blood in stool. Can be inflammatory bowel like Crohns( pain may be lower) or bowel obstruction, or superior mesenteric artery oocculsion( usually older individuals) gastritis/ ulcer, meckels diverticulum,, diverticulitis( usually left sided pain) . Go to ER today