To give context, the bowel screening programme cut off is 120 in England (lower in scotland), whereas the GP symptomatic testing cut off is 20. If you had a negative test 2 years ago, then really the most you could get in that time is small polyps. BC starts from polyps but they grow slowly, really slowly (we're talking 7+ years) before they can have potential (and not all of them do) to turn into something sinister unless you are young and have first generation family history, which I'm sure you'd know about by now.
Bowel changes mid cycle are normal due estrogen increasing in the lead up to ovulation = estrogen increases motility. Again, around the time of menstruation or hrt withdrawal bleed, loose stools normal due to sudden drop in hormones. Peri is a time of widely fluctuating hormones, bowel changes are normal and you'd only need to get it checked out if it carried on foe weeks and diet changes made no difference.